cashback.sol

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

contract CashbackContract {

  address payable shop;
  uint256 public percentage = 10; // 10%

  constructor(address target) {
    shop = payable(target);
  }

  receive() payable external {
    uint cashback = msg.value * percentage / 100;
    uint finalValue = msg.value - cashback;

    (bool successTx,) = shop.call{value: finalValue}("");
    require(successTx, "Failed to send money");

    (bool successCashback,) = msg.sender.call{value: cashback}("");
    require(successCashback, "Failed to cashback");
  }
}