condition type picking wrong value

Dear All,

We have created a Sales order where system has picked the internal price from material master which is having mvg price as 500

During return sales order for that particula material also internal price(VPRS) is 500

But after that our people have changed the material master price in MR21 which has resulted the price increase upto 1100

While raising the credit memo now system is picking the 1100 not the 500......please suggest.....lot of docs got posted