Expenses & bills
Purchase orders and vendor credits
Order from a vendor, turn the order into a bill, and record credits a vendor gives you.
A purchase order records what you've ordered; it doesn't affect your books. When the goods and the vendor's bill arrive, open the PO and convert it to a bill.
A vendor credit records money a vendor owes you (a return or an overcharge). Apply it to one of their bills to reduce what you pay.