How to pay fees for token transfers from another address

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A gas pump for your native assets, ERC-20, ERC-721 and ERC-1155 tokens.
If you’ve built or want to build a custodial exchange like Binance or Kraken, a custodial wallet service, or a custodial NFT marketplace, there’s one major headache that you’re bound to encounter: how do you pay the gas fees for sending tokens from your users’ accounts?
Astoundingly, there really hasn’t been any simple way to do this until now. As an exchange owner, every time you want to send tokens from one of your user’s accounts, you have to calculate the gas fees, send that amount to their account (a transaction that also incurs gas fees), and send the tokens from their account.
Two transactions, both of which incur gas fees, plus can result in minuscule differences of crypto “dust” remaining in their accounts. If you have thousands or millions of addresses, this method creates an enormous amount of work and fees for you as a custodial provider.

In short, a big headache. So we decided to fix that.
Gas pump: a smart contract as a user wallet
With our new Gas Pump feature, you can generate smart contracts that serve as user wallets and automatically pay for gas fees for transactions of tokens transferred from the wallets themselves.
There are two modes for this feature. The wallets automatically pay for gas fees:
By deducting them as credits from your paid Tatum plan. (Gas Pump Express)
By deducting them from a master blockchain address that holds crypto.
Smart contract implementation is available here.
Every time tokens are transferred from a user wallet, the gas fees are automatically covered either from your paid Tatum plan or from the master blockchain address you have specified.
The gas pump functionality is currently supported on the following blockchains:
Ethereum
Binance Smart Chain (BSC)
Celo
Polygon (MATIC)
Klaytn
Tron
We've created a Postman collection with all of the API calls we'll be using in this guide:
Custodial wallet.postman_collection.json
28KB
Code
Postman collection
Generate new gas pump wallets
The first step is to create gas pump smart contracts. Each smart contract functions as a wallet/user address that can accept ERC-20, ERC-721, ERC-721, and native assets on the blockchain you deploy it on.
Any combination of tokens can be transferred in a batch transaction. This is significantly more efficient and saves enormously on gas fees compared to transferring assets one by one.
Now let's generate some gas pump wallets on the Celo blockchain.
In this call, we must designate the number of wallets we want to create, whether gas fees will be deducted from your Tatum plan or a different master address, and the private key of the master address (if it is not Tatum's).
The required fields are:
owner - the blockchain address from which the gas fees will be paid. Use the address in the example below to deduct gas fees as credits from your paid Tatum plan. To deduct gas fees from a different master address, you will also need to enter a fromPrivateKey parameter.
fromPrivateKey - The private key of the address specified in the owner field from which gas fees will be deducted. ONLY include this field if you are using your own master address and NOT deducting gas fees from your Tatum plan.
batchCount - the number of user wallets you’d like to create.
feesCovered - when set to “true”, the gas fees to cover your users’ transactions will automatically be paid for by Tatum, converted to credits, and deducted from your paid Tatum plan. When omitted or set to "false", gas fees will be deducted from your own master address that you specify in the owner field.
Request
Response
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curl --location --request POST 'https://api-us-west1.tatum.io/v3/blockchain/sc/custodial/batch' \
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--header 'Content-Type: application/json' \
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--header 'x-api-key: f510b3f6-c6eb-4458-95cb-1c3d75ad3189_100' \
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--data-raw '{
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   "owner": "0x80d8bac9a6901698b3749fe336bbd1385c1f98f2",
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   "batchCount": 200,
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   "chain": "CELO",
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   "feesCovered":true
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}'
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{
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  "txId": "c83f8818db43d9ba4accfe454aa44fc33123d47a4f89d47b314d6748eb0e9bc9",
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  "failed": false
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}
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As a response, we obtain transaction hash. From this hash, we can find the addresses of the contracts - the addresses of our gas pump wallets.
Request
Response
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curl --location --request GET 'https://api-eu1.tatum.io/v3/blockchain/sc/custodial/CELO/c83f8818db43d9ba4accfe454aa44fc33123d47a4f89d47b314d6748eb0e9bc9' \
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--header 'Content-Type: application/json' \
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--header 'x-api-key: YOUR_API_KEY'
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[
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    "0xc83779f2537fd40082c031fcef91bd6557ee2a13",
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    "0xb67f45ea6c7466e25635f4154c671955df130977",
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    "0x6dcc090c52e6427938c29a5dcf03274e5bdf0630",
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    "0x29696548784515d2884fdd09ad7b4e689c56ed3f",
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    "0xf12294780cedfa51dee310cba7f3a0968d881246",
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    "0xe7b002b13a86d533abb74bbc7d5b6a16af2e6b13",
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    "0x88cf6afd1dd665abaa03cac06e6bde554e1fffd5",
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    "0xff79ff532723d56eb78c7547b53611e8dc73321e",
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    "0x17b862cf61212013602290a2a7a1ee7775b51ff6",
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    "0xba33fa91e471780db0e71771fd8af63aba6a1fb2",
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    "0xbb5747dc66e35825b9a9da800de15d6743b66b55",
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    .
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    .
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    .
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]
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You can assign these addresses to your users. They are capable of accepting CELO and any ERC-* tokens out of the box.
Transfer assets from a gas pump wallet
Your user deposited some ERC-20 or ERC-721 tokens into his account. Now it's time to move assets to another address. You don't have to send any CELO there to pay for gas; just state which assets should be sent, how much and where. Let's send an ERC-721 token with a tokenId of 100 from the address.
Request
Response
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curl --request POST \
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  --url https://api-eu1.tatum.io/v4/blockchain/custodial/transfer \
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  --header 'Content-Type: application/json' \
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  --header 'x-api-key: ' \
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  --data '{
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  "chain": "CELO",
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  "custodialAddress": "0xc83779f2537fd40082c031fcef91bd6557ee2a13",
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  "tokenAddress": "0x687422eEA2cB73B5d3e242bA5456b782919AFc85",
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  "contractType": 1,
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  "recipient": "0x687422eEA2cB73B5d3e242bA5456b782919AFc85",
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  "amount": "0",
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  "tokenId": "100",
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  "nonce": 1,
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  "fee": {
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    "gasLimit": "40000",
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    "gasPrice": "20"
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  }
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}'
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{
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  "txId": "c83f8818db43d9ba4accfe454aa44fc33123d47a4f89d47b314d6748eb0e9bc9",
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  "failed": false
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}
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As a result, we obtain a transaction hash. As you can see, we have to enter the address of the token we want to transfer, and for an ERC-721, a tokenId as well. The contractType attribute is important, as it tells the wallet which kind of token it is transferring:
contractType 0 - ERC-20 token
contractType 1 - ERC-721 token
contractType 2 - ERC-1155 token
contractType 3 - native asset - CELO
Since we specified the owner address as Tatum's address when we created the gas pump wallets in the first API call, gas fees will be deducted from our Tatum paid plan, and we do not need to included a fromPrivateKey field. However, if you specify your own master address in the owner field when creating gas pump wallets, a fromPrivateKey field containing the private key of the master address must be present.
Since the transfer endpoint is universal and neither ERC-20 tokens nor native assets have tokenIds, this value will be "0" with transfers of such tokens. Similarly, for amount, ERC-721 tokens are always one-of-a-kind, so the amount is "0".
Transfer assets from a gas pump wallet in a batch call
Simple transfers are great, but batch transfers are much more powerful. In this operation, you can basically transfer all of the assets from an address in 1 transaction. You can define different recipients for different assets, you can split the balance of the ERC-20 token into 2 different recipients and many more.
In this example, we are transferring all types of assets we have in the gas pump wallet in 1 call:
Request
Response
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curl --request POST \
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  --url https://api-eu1.tatum.io/v4/blockchain/custodial/transfer/batch \
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  --header 'Content-Type: application/json' \
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  --header 'x-api-key: ' \
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  --data '{
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  "chain": "CELO",
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  "custodialAddress": "0xc83779f2537fd40082c031fcef91bd6557ee2a13",
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  "tokenId": ["0","100","1","0"],
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  "amount": ["0.001","0","1","0.009"],
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  "tokenAddress": [
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        "0x2d7882bedcbfddce29ba99965dd3cdf7fcb10a1e",
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        "0x6d8eae641416B8b79e0fB3a92b17448CfFf02b11",
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        "0x664F97470654e8f00E42433CFFC0d08a5f4f7BC7",
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        "0"
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    ],
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    "custodialAddress": "0x4eC40a4A0dA042d46cC4529f918080957003b531",
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    "recipient": [
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        "0x8cb76aEd9C5e336ef961265c6079C14e9cD3D2eA",
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        "0x8cb76aEd9C5e336ef961265c6079C14e9cD3D2eA",
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        "0x8cb76aEd9C5e336ef961265c6079C14e9cD3D2eA",
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        "0x8cb76aEd9C5e336ef961265c6079C14e9cD3D2eA"
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    ],
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  "nonce": 1,
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  "fee": {
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    "gasLimit": "40000",
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    "gasPrice": "20"
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  }
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}'
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{
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  "txId": "c83f8818db43d9ba4accfe454aa44fc33123d47a4f89d47b314d6748eb0e9bc9",
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  "failed": false
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}
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As a result, we again obtain transaction hash as a response.
In case the asset you want to transfer does not have tokenAddress, amount or tokenId, input value 0 instead. We will take care of the rest for you.
To find out more about the API calls we have just used, please refer to our API Reference.
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Contents
Gas pump: a smart contract as a user wallet
Generate new gas pump wallets
Transfer assets from a gas pump wallet
Transfer assets from a gas pump wallet in a batch call