SO Return Discount

Hi,

We have business like 

SO Line                Unit Price      Automatic Discount 10 %              Manual Additional Discount 5 % (after 10% on sellling price)          Net Selling Price

Item A                     10                              1                                                         0.45                                                                                        8.55

once user perform SO return it calculate 15 % on unit price 10 rather than seperate as they applied on SO.

Calculation is

10*15/100

=8.50 but in actual it shld be 8.55

Any setup for modifier or any other way to tackle this issue?

Regards

Khurram