Increase Allowance of Smart Contract to Transfer a Users ERC20 Tokens

Question

First, let me apologize because I've seen this question asked multiple times but I can't figure it out still.

  function setAllowance(address tokenAddr, uint256 amount) external payable {
    IERC20(tokenAddr).safeIncreaseAllowance(address(this), amount * 10**18);
  }

  function getAllowance(address tokenAddr) external view returns (uint256) {
    return IERC20(tokenAddr).allowance(msg.sender, address(this)) / 10**18;
  }

This is my function in solidity. I'm calling it from React so within this function the caller/msg.sender is my external metamask wallet address. I want my contract to be able to send tokens on behalf of the external metamask wallet. However, the allowance does not increase. I believe my setAllowance is wrong, but it doesn't have a "owner" & "spender" parameter, only "spender" which is my contract (ie. address(this)).

So how do I know who the spender is getting the allowance from, because I want it to be the external metamask wallet, but clearly that is not happening and it's getting an allowance from something else.

I want to add in that I swapped address(this) and msg.sender, but I don't want my external wallet to have an allowance from the contract, I need it the other way around. (though in this case, the allowance did increase).

Edit 1: Im using import "@openzeppelin/contracts/token/ERC20/utils/SafeERC20.sol" and the function:

    function safeIncreaseAllowance(
        IERC20 token,
        address spender,
        uint256 value
    ) internal {
        uint256 newAllowance = token.allowance(address(this), spender) + value;
        _callOptionalReturn(token, abi.encodeWithSelector(token.approve.selector, spender, newAllowance));
    }

Edit 2: Testing it on Mumbai network (Polygon testnet)

Faucet: https://faucet.polygon.technology/

Using: Test ERC20 Plasma from this faucet on Mumbai

Testing Token: https://polygonscan.com/token/0xfe4F5145f6e09952a5ba9e956ED0C25e3Fa4c7F1

Answer 1

Are you talking about this contract?

https://rinkeby.etherscan.io/address/0x3b00ef435fa4fcff5c209a37d1f3dcff37c705ad#code

If so, I would personally just create my own interface, because the USDT implementation is kinda wild:

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

interface IERC20 {
    function balanceOf(address owner) external view returns (uint256);

    function approve(address spender, uint256 value) external returns (bool);

    function allowance(address owner, address spender)
        external
        view
        returns (uint256);

    function increaseAllowance(address spender, uint256 addedValue)
        external
        returns (bool);

    function decreaseAllowance(address spender, uint256 subtractedValue)
        external
        returns (bool);
}

You can even delete the approve() function if you won't be using it.

Note that this will NOT work the same way on mainnet, because there's a completely different implementation of an older ERC20 version.

Answer 2

Thanks to @Ismael♦ for the clue, helped me go in the right direction. Don't think I can upvote the comment cause I'm a new user (only getting flag option).

Solution:

1. Get Token ABI from etherscan
2. initialize token contract

 new ethers.Contract(tokenAddress, tokenAbi, signer)

3. call approve on token contract (giving address of ur deployed contract) THEN

4. proceed to transfer tokens from their wallet using your own contract