Fixed delivery date in Purchase requisition overwritten in ME57

Hello,

I have the following problem:

We have fixed a Purchase requisiton (EBAN-FIXKZ) on a certain date.

At this point, this Purchase requisition has no Source of Supply assigned yet.

Then we create the Purchasing Inforecord (with a Planned delivery time: EINE-APLFZ) and Source list.

Then we process the Purchase requisition in ME57:

Once we assign the Source of Supply in ME57, the Delivery date (EBAN-LFDAT) of the Purchase requistion is recalculated using the Planned delivery time from the Inforecord.

We DON'T want the Delivery date to change during ME57 (because we fixed the Purchase requision).

Is there any solution (customizing, user exit) to prevent the Delivery date from being updated in ME57 for fixed Purchase requistions?

Thanks in advance for any reply,

Kind regards,

Bart