Automatic cost adjust


I have a question about Automatic cost adjusted.

for example,
1 Purchase document with item qty 1 and cost amount is 1000
Item already Sold
then create Purchase Invoice again with qty 1 and cost amount is 1200
After that, i created Purchase Credit Memo and applied to Purchase invoice with cost amount 1000,
but when i check the G/L Entry, In the purchase credit memo have 1 adjustment for cost 200.

so, Finish Good account is not correct.

Can anybody to help me solve this case? Urgent