Under a transaction For review in Banking, money in now lists Payment for an open invoice and money out lists Payment of an open bill: invoices and bills that still have at least that amount owed. Choosing one records the payment on the invoice or bill, into or out of the account the transaction is in, and matches the transaction to it. A smaller amount is recorded as a part payment.
An invoice or bill owed exactly the amount is suggested when the description names its customer or supplier, or when no other one is owed that amount, so Add all suggested can record it. Entries already in the books whose customer or supplier the description names are listed first. Undo removes the payment again.