Mixed disposition in a single Usage Decision

Hi all,

I am new to SAP and so I am looking for some advices

The question is the following:

The quality lot is generated by a Purchase Order.

I want to understand if it is possible to set up the system in order to define with one single Usage Decision that I want, in example, accept 80 pz out of 100 and move them from stock Q to "free" stock plus I want to send back to my vendor the 20 pz that I do not want to accept.

So far I see I can set 80 pz into "free" and 20 into "blocked" but this split is not possible when I chose to send some material back to the vendor.

(I get error message M7 136)

thank you very much

valentina