while doing miro / mir7 system is picking value from PO instead of GRN / GRIR

[PD11]

Hi All

we have a customization in material GRN process to address the rebate process. the functionality is as below

PO is raised for 100 qty @20  - Value is 2000

GRN happens thru our custom program, it considers the quality parameters and then arrives the rate based on the material supplied.  for eg: it is derived as 15

then the custom program calls the GRN bapi with the rebated value and GRN happens @15 in this scenario the value is 1500 not 2000

Now if I go to MIR7 /MIRO system creates the invoice for 2000 even my GRIR is 1500.  

Is this can be restricted? please suggest