F110 - How to group two payments for a single costumer

Hello experts,

I'm having an issue here. The user has 14 invoices, 11 of them are DR document type, 3 are DM doc. type. All of the 14 invoices have payment method 4. When we create the proposal for this costumer, two payments are created, one for all the DR invoices, and another one for the DM invoices.

What the business want is to be able to create only one payment, including both DM and DR invoices. I tried looking into FBZP for payment method "4", both in company code and country. I flagged "Single payment for marked item" and nothing changed, 2 different payments are still created for the same customer.

Also I looked into the customer master data, on the "Payment Transaction" tab, flagged the "Single payment" but it works exactly the other way, one payment document per invoice, which I do not need. I saw something about Grouping keys, created one with GL Account, Customer and Posting key but did not work, still two payments for the same document.

Is there anything else that I can try? Any help will be appreciated. Please let me know if something is unclear.

Best regards and thanks in advance,

Ivan