Purchase return cost effect on invent trans

Dear Axians,

I have posted one purchase return order as below

Qty = 10 pcs

Purch Price = 25.00

Total = 250.00

In my case, invent trans need to be as below

Qty = -10

Costamountposted = 10.00

But I dont understand, in my case some purchase return have invent trans as below

Qty = -10

Costamountposted = 8.70 (values are dummy for eg)

Costamountoperation = 1.30 (values are dummy for eg)

I checked the voucher and found that 1.30 is posted by posting type "Purchase expenditure for product". 

So in my case, if i had made a purchase of 10 pcs with 250.00 and made a return purchase order of 10 pcs with 250.00 itself. Inventory qty balance will be 0 pcs, but value of 1.30 exists there. This is creating so many issues in inventory values, costing etc.

Is this normal situation and why ?. How to tackle this case.

My costing method is weighted avg dated.

Thanks in advance.