Money out mirrors money in: the purchase order (what you asked for), the bill (what you owe), and the bill payment (what you sent).
Entering a bill
Open Bills and create one. Pick the vendor, the date and the due date, and add lines with their expense categories. If the bill arrived as a PDF, attach it: the document stays with the bill forever, so the original is always one click from the entry it explains. A purchase order you already entered can fill the bill in for you; if that bill is later voided or deleted, the order gets those quantities back so the corrected bill can be made from it.
Paying
Pay bills lets you settle one or many at once: tick the bills, choose the account the money leaves from, and record it. Paying by paper? kBooks prints checks on standard check stock, and the check register under Transactions keeps the numbering straight.
When the payment clears the bank, the feed line matches the payment you recorded instead of creating anything new, so nothing doubles.
A check number stays used once it is written. Delete the check or the bill payment later and the next number suggested on that bank account is still past it, so two paper checks never share a number.
Got something wrong on a payment? The pencil on a Payments made row (or on the payment inside a bill) opens the payment itself: change its date, method, check number, memo, or the account it came out of. A printed check keeps its number and bank (void it from Print Checks to change those), and what the payment paid stays as it is - void it and pay again for that.
A paid bill opens for editing like any other; only the vendor stays, and the total cannot drop below what is already paid. A partly received purchase order edits too: lines that have arrived or been billed keep their account and cannot go below what came in, and everything else - memo, dates, prices, new lines - is free.
No vendors yet? Start a bill anyway: type the vendor's name in the bill and it is added to Contacts as you go. Paying by check suggests the next check number on the bank account you pay from.
Recurring costs
Bills that repeat (rent on your office, software, insurance) can be memorized so each period's copy appears on schedule, ready to confirm rather than retype.
Keeping an eye on what you owe
The Overview shows what is due and what is overdue, and the payable aging under Reports breaks it down by vendor and by how long it has waited. Each vendor's page carries their whole history: bills, payments, documents and notes in one place.