Purchase order repricing automatically when changing some information

hi Everyone,

We have the issue that when we change the branch-plant in a stock PO, the system is automatically re-pricing the PO line. Is there a way to "lock" the Po price in a PO? I tried processing option tab 2 - 5. cost protection set to 1 = disable cost field, but that only prevents manual entry, not automatic update while what I'm trying to achieve is to lock the unit cost (for both manual or automatic update) while allowing other changes. Any suggestion?

Regards

Nicolas