Swap ERC20 tokens from ERC721

Smart Contracts
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1

Hello to all of you. We are struggling with this:

He have deploy an ERC20 token. Everything works perfectly. Now we want holders to spend ERC20 tokens to mint an ERC721 . So far so good

On mint function we want some percent to send to charity wallet as ETH or BNB. We implement the Uniswap Factory ( or Pancake for BSC) and we try to call the swaptokens for ETH but we get SafeMath: subtraction overflow Our ERC721 has the receive function and we gave allowance to the contract to spend the tokens. Also try allowance with the user.

Any help??

2

Hi, welcome! :wave:

If possible, please share your contract code at here.

3

Hi and thanks for the reply.

This is the debug error

The contracts is in BSC test net

4

Ohhhh, this tool is really very good. And it seems like when you want to swap token, it will call transferFrom, so you need to have a check whether the caller has enough approval amount.

5

We check before the swap. I think it has to do with the transferFrom on ERC721.

@dev See {IERC721-transferFrom}.

function transferFrom(address from, address to, uint256 tokenId) public virtual override {
    //solhint-disable-next-line max-line-length
    require(_isApprovedOrOwner(_msgSender(), tokenId), "ERC721: transfer caller is not owner nor approved");

    _transfer(from, to, tokenId);
}

We call this function before the swap

    token.approve(address(uniswapV2Router), tokenAmount);

    // make the swap
    uniswapV2Router.swapExactTokensForETHSupportingFeeOnTransferTokens(
        tokenAmount,
        0, // accept any amount of ETH
        path,
        devWallet,
        block.timestamp
    );

We are calling this function in ERC721 contract and it has its own transferFrom but with token id.

Do we need to overide the transferFrom and call transferFrom on ERC20 ?
Does the router call back the msg.sender () ? <-- sender is ERC721

This is the function on PancakeRouter

function safeTransferFrom(
        address token,
        address from,
        address to,
        uint256 value
    ) internal {
        // bytes4(keccak256(bytes('transferFrom(address,address,uint256)')));
        (bool success, bytes memory data) = token.call(abi.encodeWithSelector(0x23b872dd, from, to, value));
        require(
            success && (data.length == 0 || abi.decode(data, (bool))),
            'TransferHelper::transferFrom: transferFrom failed'
        );
    }
    function safeTransferETH(address to, uint256 value) internal {
        (bool success, ) = to.call{value: value}(new bytes(0));
        require(success, 'TransferHelper::safeTransferETH: ETH transfer failed');
    }
}
6

We found the solution

It was on approval as @Skyge mention

    Atoken.approve(address(this), tokenAmount);
    Atoken.approve(address(uniswapV2Router), tokenAmount);
    Atoken.approve(ATokenAddr, tokenAmount);
    Atoken.increaseAllowance(address(uniswapV2Router), tokenAmount);
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