Dear Friend,
Considering the situation, I suggest you to take the total credit on both the materials in your cenvat account and then return the rejected material by raising excise invoice on payment of duty equal to the credit taken under Rule 3(5) of CCR,2004.
The above procedure not only solve your problem before you but also facilitates the supplier to take credit under Rule,16 of CER,2002 under Market returns.
If it is not so, how you satisfy the supplier for the amount of duty paid by him by sending the rejected material without duty paid invoice.
Another alternative available to you is to return both the materials under the cover of the supplier's invoice as it is so that the supplier can take credit hassle free and subsequently, he would be dispatching the required material under a single invoice which is again subject to condition of amounting to manufacture etc under Rule 16 which is again going to be a complicated one
Hence, I suggest to proceed with the first advise.
Best Regards
Suryanarayana