To pay a purchase invoice to have several ways available
- Cash Accounts tab - Spend Money with Account = Accounts Payable + Supplier + Invoice
- Suppliers tab - click on the blue balance for the supplier - click View for the invoice to be paid and click on the Spend Money button.
However you would only use Supplier & Purchase Invoices if you were purchasing on credit - deferred payment. When you pay immediately for the purchase then it is a cash purchase - just use Spend Money directly. Refer to the Guides