[mothered]

I want to social engineer a mouse by buying a returned mouse from razer and sending back my old razer mouse.

You'd be using the "sealed box method". This Is on the grounds that the contents are not visible as per the manufacturer's packaging.

The objective Is to replace the new Item with the old, and not show any signs of tampering. As such, the store person will place It back In their Inventory and Issue a refund/replacement thereafter.