If an invoice has been created in accounts receivable and has been printed it can’t be cancelled.
To reverse the error – In Accounts Receivable, create a Receipt as if you have received payment for the invoice. Post it via the paying-in slip.
In FMS | General Ledger | Manual Journal Processing – create a Cash Book Journal Debit to the bank to reverse out the payment.
Reconcile the two items in General Ledger | Bank Reconciliation – Add a statement and choose the two cancelling items and save.