Open items are what the aged reports show. Applying payments to invoices, and unapplying when it was wrong, is a daily finance task worth doing well.
When posting a cash receipt journal, use Apply Entries on the line to pick the invoices it pays; or set "Applies-to Doc. No." for a single invoice. Posted but unapplied? Open the customer ledger entries, select the payment, Apply Entries, set Applies-to ID on the invoices, Post Application. The Cash Receipt Journal's Apply Automatically matches by document number and amount.
Applied to the wrong invoice: open the entry, Unapply Entries, confirm; both are open again. Business Central records every application and unapplication in the detailed ledger entries, so nothing is lost and the audit trail shows it. Keep the ledgers applied up to date and the aged debtors report is right without a spreadsheet.
