Transfer from contract address to sender

Question

I am coding a crowdfunding contract where senders have two options. First, payIn which invokes a transfer to the contract's address. Second, withdraw which invokes a transfer from the contract's address to the sender. However, I cannot transfer to the contract's address because it is not payable. How can I make the contract payable?

pragma solidity >=0.7.0 <0.9.0;

contract Crowdfunding{
    // Crowdfunding settings.
    address payable owner;
    uint256 goal;
    bool isOpen;
    
    // Store the crowd funding values
    mapping(address => uint256) public crowd;

    constructor(uint256 _goal) {
        owner = payable(msg.sender);
        isOpen = true;
        goal = _goal;
    }
    
    function payIn() public fundingIsOpen payable {
        address(this).transfer(msg.value);
    }
    
    function withdraw() public fundingIsOpen payable {
        address(msg.sender).transfer(crowd[msg.sender]);
    }

...

Answer 1

Lots of ways to go about this, one that would work is

function withdraw() public fundingIsOpen payable {
    payable(msg.sender).transfer(amount);
}