In Cash -> Vouchers -> Reverse Receipts (at least, perhaps more), filter on Customer Number does not work.
When you enter a customer number, you get all receipts loaded.
https://github.com/ledgersmb/LedgerSMB/commit/dc8d3f4d32f8e7c5b8b6e0ffacb4ef41758cc0a1
https://github.com/ledgersmb/LedgerSMB/commit/dc8d3f4d32f8e7c5b8b6e0ffacb4ef41758cc0a1