I don't actually think we gain much from holding Torreira back.
First, I would trust Simeone/Atletico more than Arteta/Arsenal to highlight Torreira's value. He won't be getting game time here, and we'll be looking to move him again next summer with little interest.
Second, if we punk Atletitco by signing Partey on the last day, and then pull out of a transfer, it will look mean spirited. That certainly won't help our negotiations in the future or other clubs' desire to work with us.