MAP becomes value less when I use 122 movement type

Hi Guys

AS per the process mapped by our SI, when I use the 122 movement type to return to vendor, my current stock value becomes zero for the remaining stock at the storage location. How to prevent it. E.g My stock at storage location before 122 was 100 at MAP 10.00 each. if my POvalue for the 80 numbers is 100, then on using 122 the value of the remaining 20 becomes zero. How to prevent this.

Regards

Pradeep/ICF