Hello
BAdI MD_PURREQ_POST should not be used to change any kind of data. See the following information provided on the BAdI documentation:
"You cannot, however, use the Business Add-In to change the data of the
purchase requisition itself. The purchase requisitions are always posted
in the communicated form."
I suggest you to try using BAdI MD_PURREQ_CHANGE, which is called by MRP and it allows you to change purchase requisition data.
You may also find another BAdIs for MRP on the document BAdIs for MRP
BR
Caetano