How to do infinite approve/transferFrom in Solidity

Question

I want to write a contract that buys back an ERC20 Token using USDC. (in REMIX IDE)

I want to understand what is the best practice for paying out the USDC in return for the ERC20 that was sold to the contract.

Right now, I am calling a

usdcAddress.transferFrom(address(buyer), msg.sender, price);

It just dawned on me that I need to the buyer wallet address to call APPROVE before I can call the transferFrom which is something I cannot do in production. Only the msg.sender will be able to call approve via the web3.

What is the best practice to implement this scenario.

Answer 1

Well this may not be the best way but you can do this in 2 steps instead of doing this in 1 step . Example : Step1 : Seller deposits the ERC20 token to smart contract first. Step2 : Buyer take ERC20 token from smart contract and in same function buyer will be sending USDC to the seller as well.

Answer 2

What I ended up doing is deciding to just regularly send in USDC to my contract, and just call transfer() from there. If there are any other suggestions, it's still welcome. Thank you

Answer 3

The best practice for implementing this scenario would be to have the buyer first approve the contract to transfer the USDC from their account before calling the buyback function. This way, the contract can call the transferFrom() function without needing the buyer's address.

Here is an example of how this could be implemented:

The buyer calls the approve() function on the USDC contract, passing in the address of the buyback contract and the amount of USDC they wish to use for the buyback. css

usdcAddress.approve(address(buybackContract), price);

The buyer then calls the buyback function on the buyback contract, passing in the address of the ERC20 token they wish to sell and the amount of tokens they wish to sell. scss

buybackContract.buyback(tokenAddress, tokenAmount);

The buyback function in the buyback contract checks that the buyer has approved the contract to transfer the necessary amount of USDC, then calls the transferFrom() function on the USDC contract to transfer the funds from the buyer's account to the contract's account. less

require(usdcAddress.allowance(msg.sender, address(buybackContract)) >= price, "Not enough approved USDC."); usdcAddress.transferFrom(msg.sender, address(buybackContract), price);

Finally, the buyback function calls the transfer() function on the ERC20 token contract to transfer the bought tokens to the contract's address.

tokenAddress.transfer(address(buybackContract), tokenAmount);

It is important to note that the buyer has to trust the contract enough to approve it to transfer their funds, and also the contract should be designed to ensure the buyer's funds are safe and the buyer can get the correct amount of tokens they paid for.

Additionally, it's important to consider the gas fees and make sure the contract has enough balance to pay the gas fees before executing the transaction.