Undo shippment - Shipped Qty. not returned

Dear all,

here is the scenario (kinda complicated, but i'm tryin' a few stuff on a demo database): 

I create a sales order. 

I post the shipment, but not the invoice. 

I do some inventory adjustement with the phys. Inventory journal.

I create a sales return order. 

I realise i have to cancel the posted return receipt and the posted sales shipment.

I can't undo the shipment on both of them :

i have "shipped qty. not returned error message" when i try to undo shippment on the posted sales shipment.

I have a "remaining quantity must be equal to "10" in item ledger entry. ... Current value is '0' "

Anyone could explain me where is the problem ?