Wrong MAP because of incorrect rebate calculations incase of GR reversal

Dear Experts,

Hope all are doing good.

Currently we are facing few issues with our Rebate calculation setup in PO processing. The way how system calculating rebate is leading to incorrect MAP(Moving Avg. Price). Incase of GR reversals rebate value sign is not considered(It should be +ve but instead it is -Ve). let me give you guys a small example.

Lets say,

1) We bought 10 cartoon for pepsi for 100 rs and rebate is 10%.

                      -> Price : 100 rs

                      -> Rebate: -10 rs

                      -> MAP = (100-10)/9 = 9rs.

2) We canceled above GR

                      -> Price: -100 rs

                      -> Rebate: should be +10 but instead it is '-10'.

3) We did GR again.

                       -> Price: 100rs

                       -> Rebate is calculated as +10 to correct the impact caused by step 2. this is leading into wrong MAP as shown below

                       -> Map = (100 + 10)/10 = 11. In reality is should be 9.

what we want is, system should consider GR and GR reversal in the process of rebate calculation. If some one can through some light it would really help me in fixing this issues.

Thanks in advance. Hope i am clear enough.

Thanks,

Rakesh.