There are 2 ways to create credit memos. On the one hand, you can create a credit memo without reference to an order under Accounting/Invoicing and fill this credit memo freely with data and content, or you can enter a partner for orders.
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In this case, the commission stored is automatically included in the product in a way that is not visible to the customer and a credit memo is created when the invoice is issued.
For each organization, you can define a percentage commission. However, you can also change the commission manually for each order.  
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Once you have invoiced the invoice, you will find the credit memo in the outgoing invoices and can then invoice them with just a few clicks just like any normal invoice.