In Opencart Marketplace RMA system, the admin can update the stock quantity of returned products in the store.
On the Invoice page, the admin will have the Return Quantity button.
Click on the Return Quantity button. This will update the stock value of the product on the store in real time. The same will be reflected in the return request list too as per the image.
Please do contact us for any further queries or requirements. You can also check our Opencart Extensions for additional features.