Prevent vendor mat number from being changed automatically

Hi experts,

we use SRM for creating shopping carts and create purchase requisitions in R/3. The users are asked to add the vendor material numbers even if they create freetext shopping carts. Now I create a PO based on the freetext PR, and as soon as I enter the vendor the vendor mat number (IDNLF) disappears. So it would be necessary to copy it from the PR into the PO.

Is there a possibility to prevent this field from beeing changed while changing the vendor? Or other ideas how to solve this problem?

Thanks and regards

Hubert