USDT transfer on my contract fails

Question

I am now developing a ICO smart contract which receives USDT tokens from other people and withdraw.

My withdraw function in contract is below.

function withdraw() external onlyOwner onlyContractHasMoney {
        // here, contract will send usdt to _wallet
        uint balance = _usdtToken.balanceOf(address(this));
        _usdtToken.transfer(_wallet, balance);
}

And here is USDT transfer code that I wrote for localhost test purpose.

function transfer(address _to, uint _value) public onlyPayloadSize(2 * 32) {
        console.log("msg sender of transfering is %s", msg.sender);
        uint fee = (_value.mul(basisPointsRate)).div(10000);
        if (fee > maximumFee) {
            fee = maximumFee;
        }
        uint sendAmount = _value.sub(fee);
        console.log("balance of msg sendger is %s and transfering value is %s, receiving address is %s", balances[msg.sender], _value, _to);
        balances[msg.sender] = balances[msg.sender].sub(_value);
        balances[_to] = balances[_to].add(sendAmount);
        if (fee > 0) {
            balances[owner] = balances[owner].add(fee);
            Transfer(msg.sender, owner, fee);
        }
        Transfer(msg.sender, _to, sendAmount);
        console.log("there is no problem until yet");
}

And my javascript withdraw scripts is below.

const hre = require("hardhat");
const ICO = require('../../clients/artifacts/contracts/ico.json');

async function main() {
    const icoAddress = ICO.address;
    const icoABI = ICO.abi;

    const [signer] = await ethers.getSigners();

    const ico = new hre.ethers.Contract(icoAddress, icoABI, signer);

    await ico.withdraw();
}

main()
  .then(() => process.exit(0))
  .catch((error) => {
    console.error(error);
    process.exit(1);
  });

But when I run the script, I am getting this error.

As you can see, USDT transfer transaction reverted because of UNPREDICTABLE_GAS_LIMIT, not usdt transfer code itself. I am not sure how can I resolve this issue. Do you have any idea to solve this problem? Please help me!

Answer 1

The error is saying that can't estimate the gas limit. Just add hardcoded one ase that:

await ico.withdraw({ gasLimit: 3000000 });

Answer 2

I think the contract first has to approve the transcation, then you can call the method tranfer().

Answer 3

Without looking at the rest of the code the only thing that comes to mind is that you're using a wrong interface for _usdtToken. It's a common error since USDT doesn't conform exactly to ERC20.

Try adding this:

interface IUSDT {
    function transfer(address, uint256) external;
}

and do this:

function withdraw() external onlyOwner onlyContractHasMoney {
        // here, contract will send usdt to _wallet
        uint balance = _usdtToken.balanceOf(address(this));
        IUSDT(address(_usdtToken)).transfer(_wallet, balance);
}

Answer 4

i was also facing the problem and i came to know that some of the token's aren't fully compliant with the ERC20 standards.

i used safeERC20 to get rid of the error linking an article about detailed explanation of the error and the solution.

https://medium.com/@JohnnyTime/why-you-should-always-use-safeerc20-94f44aa852d8