When reviewing the Bills in AP, some of the invoices are missing the Pay link.
In Accounts Payables, clicking on Bills, brings up a list of invoices which may have a status of paid, partially paid, or posted. Normally, when an invoice is posted or partially paid, there is a link, 'Pay', on the invoice line which, when clicked, opens the Select Bills to Pay screen, with that invoice already selected. This is a shortcut to paying individual invoices. However, in some cases, that Pay link is absent. When the original invoice balance is a negative, the Pay link will not be available. Even if the credit lines of the invoice have been paid (credits taken) and the balance owing on the invoice is not a negative amount, the Pay link still doens't show up because of the original credit balance. The only way to pay this bill will be to use the AP / Select Bills to Pay function instead of the shortcut.