Unregister - Return sales order

Hi all,

I create Item arrival 2 times for return sales order (1 line = 1 item number) but I create packing slip and invoice for the first item arrival.

So, the inventory transaction occur 2 lines with 2 status: Purchased and Registered

The status of this sales order is still Open

I want to unregister the line in return order but all registered/pick button is invisible in all screen (sales order/ RMA)

Please help me unregister and change return order status to invoiced.