Transfer of ERC-20 tokens fails, even though smart contract has enough token balance

Question

I am trying to develop a buy function for my token smart contract, but it fails on the transfer:

transfer(msg.sender, amountToBuy);

It fails with the Error: 'ERC20: transfer amount exceeds balance', even though the contract has enough tokens in its balance:

Answer 1

Ok I found the Answer. Instead of just doing transfer in the code, I create an object out of the IERC20 Interface.

This is needed for a smart contract to transfer its own tokens:

IERC20(this).transfer(msg.sender, amountToBuy);