Free items in purchasing contract

Hi all,

Our POs are contract based only.

Some of the items are "free of charge" ones. Meaning then when buying item X with full price, we receive free of charge item Y.

Now, we don't want the buyer to choose this free item Y in the PO, but we need the information to come for the contract.

In the contract - no "free item" flag exists.

How can we acheive this goal?

(I've read some suggestions here referring to condition type NR00, but I don't find it relevant for us).

Thanks,

Limor