How do I proceed if I want to map a purchase on account where the purchased goods are returned later?

If the goods are issued/sold to the customer with an invoice, you can use a different payment method, such as "purchase on trial" or invoice 40 days.

Store the goods via the goods receipt as a return, if the customer does not keep the goods.

Then create a cancellation invoice (complete cancellation) or a credit note. The credit note then fully offsets the invoice. A cash flow takes place only if the customer keeps and pays for the goods.