contract InvoiceManager {
    struct INVOICE {
        bool valid;
        ...
    }

    mapping (uint => INVOICE) invoices;

    event InvoiceCreated(uint id, ...);
    event InvoicePaid(uint id, ...)
    ...

    function createInvoice(uint id, uint sum, ...) {
        ...
        INVOICE storage invoice = invoices[id];
        require(!invoice.valid);
        ...
        emit InvoiceCreated(invoice, ...)
    }

    function payInvoice(uint id) public payable {
        ... process payment ...
        emit InvoicePaid(id, ...)
    }
}