function cancelPayment(uint64 paymentId) external onlyPayer(paymentId) {
        require(payments[paymentId].payeeBondPaid == false);
        transferTokens(
            address(this),
            msg.sender,
            payments[paymentId].amount,
            payments[paymentId].isEthPayment
        );
        _deletePayment(paymentId);
    }