ApproveUSDT function not working internally, how do I fix this?

Question

I’m experimenting with a presale contract based on the BSC testnet, and in the contract I’m attempting to achieve the purchase of my token with USDT.

Using my own code within the contract itself, I cannot get approval to go through properly for the contract to spend my USDT. However if I use the approve function within the “write contract” section of the USDT token on BSCSCAN, it does work - and I can buy tokens after approval went through.

Example of my approval code which does not work:

function approveUSDT() public {
            
IERC20 usdtToken = IERC20(0x337610d27c682E347C9cD60BD4b3b107C9d34dDd);
    
require(usdtToken.approve(address(this), tokenAmount), "Approval failed");
            
emit ApprovalSuccessful(msg.sender, tokenAmount);
    
}

What am I missing here?

Answer 1

If I understood correctly, you want the contract to withdraw usdt from your wallet and send you back tokens in exchange for that USDT.

If that's correct, then your doing it wrong.

You need to go to the USDT contract and approve your deployed contract to spend your USDT. On etherscan, you can put the USDT address in and navigate to contract and there's write functions that allow you to do this.

This would permit your deployed contract to withdraw USDT from your eg metamask wallet.

Your deployed contract itself, would have a function such as this (I code in vyper but solidity logic would be similar).

@external
def buy_tokens(amount: uint256): 
   ERC20(USDT).transferFrom(msg.sender,self,amount)
   token-amount: uint256 = some-logic-to-calculate-fx-between-usdt-and-your-token-fx-to-usdt
   ERC20(self).send(msg.sender,token-amount)

Answer 2

USDT can be a weird token. It's better if you use the SafeERC20 library by openzeppelin. There's a function called safeIncreaseAllowance, which is basically approve, that solves some issues.

import '@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol';
using SafeERC20 for IERC20;


function approveUSDT() public {
    IERC20 usdtToken = IERC20(0x337610d27c682E347C9cD60BD4b3b107C9d34dDd);
    usdtToken.safeIncreaseAllowance(address(this), tokenAmount);
    emit ApprovalSuccessful(msg.sender, tokenAmount);
}

Answer 3

You are trying to call approve function of token contract in another contract("Pre Sale"). This wont work because it will try to approve Pre Sale contract for Pre Sale contract.You are trying:
Owner: Pre Sale contract
Spender: Pre Sale contract

So, you need to call approve function of token contract in token contract. You can do this on the frontend with ethers.js package or any other frontend package. This way:
Owner: user accounts
Spender: Pre Sale contract