For Substitutue Items

Scenario:

An few Item is received in inventory after some days later a particular item is return to supplier now the
supplier hasn't the same item in his stock so he gave another item in replace of original.

 For this i do the below steps but i didn't working. Some one guide me what am i missing?

I did  " click on Shipments button>>click on receiving controls and check if allow substitute receipts check box is checked"
 but i can't change the item. It gets the PO item.



Thanks
Farhaj Ahmed Khan