Dear,
MRP is based on the stock levels.For example PD type - its only based on the demand from maintenance orders etc... and VB type -Its based on the reorder point level.etc..
What would be the situation if your user posts goods issue on back dates?Still you want MRP to be carried out? as per you requirement.Its not possible to have only to have P.R for the GI qty.
MRP Concept:
MRP is the material planning tool in SAP, which will take care of all aspects of a material and is the completely based on the master data of the material.
MRP considers teh current stock, current requirements, and open purchase requisition/Purchase orders etc.. as per your config settings at MRP .
Regards,
Pardhu