Hello JanC!
Let's record and apply the single payment you received to different invoices.
You can record the payment as a deposit, and allocate the amounts to different customers by tagging their names and using the Debtors account. In this way, the deposit will turn into credits which you can use to pay their invoices.
Let's say you received €1000 to pay invoices for Mary (€250), John (€500), and Nick (€250). You'll want to record it like in the screenshot.
When you pay their invoices, you can click the Receive payment button on the Invoice page. Then, on the Receive Payment page, select the deposit in the Credits section.
We're just around if you have other questions.