Escrow contract for Optimistic relay v2

EscrowRelay.sol

contract Escrow {
    event Payment(
        address builder,
        address feeRecipient,
        uint256 amount,
        uint256 nonce,
        uint256 expiry,
        bytes32 blockhash
    );

    struct Account {
        uint256 timelock;
        uint256 balance;
        uint256 nonce;
    }

    mapping(address => Account) public accounts;

    address public immutable admin;
    uint256 public immutable timelockDuration;

    constructor(address a, uint256 t) {
        admin = a;
        timelockDuration = t;
    }

    // builder deposits eth into their account
    receive() external payable {
        accounts[msg.sender].balance += msg.value;
    }

    // escrow executes payment
    function pay(
        address feeRecipient,
        uint256 amount,
        uint256 nonce,
        uint256 blockNumber,
        bytes32 blockHash,
        bytes memory permission
    ) public {
        // Checks
        bytes32 message = keccak256(abi.encodePacked(feeRecipient, amount, nonce, blockNumber, blockHash));
        address builder = ecrecover(message, permission);
        require(nonce = accounts[builder].nonce + 1, "Builder Nonce does not match");
        require(blockhash(blockNumber) == blockHash, "Blockhash does not match");

        // Effects
        accounts[builder].nonce = nonce + 1;
        accounts[builder].balance -= amount;

        // Payment
        payable(feeRecipient).transfer(amount);
        emit Payment(builder, feeRecipient, amount, nonce, blockNumber, blockhash);
    }

    // builder requests exit
    function requestExit() public {
        require(accounts[msg.sender].timelock == 0);
        accounts[msg.sender].timelock = block.number + timelockDuration;
    }

    // builder confirms exit
    function confirmExit() public {
        Account memory account = accounts[msg.sender];
        require(account.timelock != 0 && block.number >= account.timelock);
        delete accounts[msg.sender];
        payable(msg.sender).transfer(account.balance);
    }
}