MerchantPayable.sol

contract MerchantPayable {

	enum Role {Client, Merchant, Owner}

	mapping (address => Role) public addressToRole;
	mapping (bytes32 => bool) public paymentSettled;
	mapping (bytes32 => bool) public paymentIssued;
	
	uint256 nonce;

	event PaymentIssued(
		address client, 
		address merchant, 
		uint256 value, 
		uint256 timeStamp, 
		bytes32 paymentHash,
		uint256 nonce
	);

	event PaymentSettled(bytes32 paymentHash);

	modifier onlyClient {
		require(addressToRole[msg.sender] == Role.Client);
		_;
	}

	modifier onlyMerchant {
		require(addressToRole[msg.sender] == Role.Merchant);
		_;
	}

	modifier paymentHasNotBeenSettled(bytes32 paymentHash) {
		require(paymentSettled[paymentHash] == false);
		_;
	}

	modifier paymentHasBeenIssued(bytes32 paymentHash) {
		require(paymentIssued[paymentHash] == true);
		_;
	}


	function pay
	(
		address _merchant, 
		uint256 _value
	) 
		onlyClient 
		public 
	{
		// the actual token goes to the owner
		token.transfer(owner, _value);
		
		// increase nonce
		nonce = nonce.add(1);

		// generate a hash as the unique identifier for this payment
		// @notice may NOT be unique. Can be somehow forgeable!!!
		bytes32 _paymentHash = keccak256(abi.encoded(msg.sender, _merchant, _value, now, nonce));

		// all the requirements
		require(paymentIssued[_paymentHash] == false);
		require(paymentSettled[_paymentHash] == false);

		// set payment to issued
		paymentIssued[_paymentHash] = true;

		// record the payment as settlable receipt for the merchant
		emit PaymentIssued(msg.sender, _merchant, _value, now, _paymentHash, nonce);	
	}




	function settle
	(
		bytes32 _paymentHash
	) 
		onlyMerchant 
		paymentHasNotBeenSettled(_paymentHash) 
		paymentHasBeenIssued(_paymentHash) 
		public 
	{
		// set payment to settled
		paymentHasBeenSettled[_paymentHash] = true;

		// settle the payment with the exact unique identifier
		emit PaymentSettled(_paymentHash);
	}

}

Status of a Payment: None --> Issued --> Redeemed

TODO:

1. Register Role for a user
2. Instantiate ERC20 token for payment