Payment.sol

//based in zepplin

pragma solidity ^0.4.23;


import "../math/SafeMath.sol";


/**
 * @title PullPayment
 * @dev Base contract supporting async send for pull payments. Inherit from this
 * contract and use asyncSend instead of send or transfer.
 */
contract PullPayment {
  using SafeMath for uint256;

  mapping(address => uint256) public payments;
  uint256 public totalPayments;

  /**
  * @dev Withdraw accumulated balance, called by payee.
  */
  function withdrawPayments() public {
    address payee = msg.sender;
    uint256 payment = payments[payee];

    require(payment != 0);
    require(address(this).balance >= payment);

    totalPayments = totalPayments.sub(payment);
    payments[payee] = 0;

    payee.transfer(payment);
  }

  /**
  * @dev Called by the payer to store the sent amount as credit to be pulled.
  * @param dest The destination address of the funds.
  * @param amount The amount to transfer.
  */
  function asyncSend(address dest, uint256 amount) internal {
    payments[dest] = payments[dest].add(amount);
    totalPayments = totalPayments.add(amount);
  }
}