UsageExample.sol

// SPDX-License-Identifier: MIT
pragma solidity ^0.7.0;

/**
 * @title NiceGuy
 * @dev Implements `freeTx` modifier to add a full transaction cost rebate to any function
 */
contract NiceGuy {

    /**
     * @dev Transfer all ether use in the contract function back to the sender
     */
    modifier freeTx() {
        uint256 startGas = gasleft();
        _;
        uint256 spentGas = startGas - gasleft() + 21000 + 16 * msg.data.length;
        payable(msg.sender).transfer(spentGas * tx.gasprice);
    }

}

/**
 * @title Button created by Marto
 * @dev Usage example of NiceGuy "freeTx" modifier to add a full transaction cost rebate to any function
 */
contract Button is NiceGuy {
    uint256 public counter = 0;

    constructor() payable {}

    /**
     * @dev Transfer all ether use in the contract function back to the sender
     */
    function push() public freeTx {
        // Make some expensive computation
        for (uint i=0; i < 500; i++) {
            counter = counter + 1;
        }
    }

    function getBalance() public view returns (uint256) {
        return address(this).balance;
    }
}