Here is my bloody workaround:
1. Create a time & material type (!) sub-sub-project. Keep all the parameters and dimensions to their default values, this will record the negative cost price “revenue” against the original financial dimensions
2. Set the sub-sub-project stage to In process.
3. Open the list of Open return orders, create a new one, and with the form personalization, add the Project ID field (I know, this is sick)
4. Choose the Project ID, it populates the customer etc.
5. Add the lines to the resulting Return order with a negative quantity. The system complains about the Line property. Add this column – Line property – with the form personalisation and choose NBillable. Do NOT set a delivery date in future.
6. From the return order header or from the list of project Item tasks, drill down into the related Sales order and update the quantity from empty to the respective negative quantity.
If you fail to do so, the warehouse worker will experience an error message about the quantity on an attempt to receive the goods at the mobile scanner.
7. Give the RMA number (maybe the RMA labels too?) to the customer
8. The goods arrive, and the warehouse worker may receive them as usual with the menu Reception retour.
9. [Not a mandatory step] Post the delivery note (aka packing slip) against the return sales order
10. Once upon a time, create a new T&M project invoice proposal in the below very special mode Sales order line = Packing slip (depends on step 9) or All, press Search.
11. This will create a zero T&M project invoice with the returned items.
12. Approve and post the invoice, this will negate the original item cost, produce project transactions and ensure the proper stock value. The “invoice” must not be sent to the customer.