Stock Transfer Between Storage Locations

Hi,

I have activated the function for issuing storage location in STO (Materials Management - Purchasing - Purchase Order - Set up Stock Transport Order - Activate Stock Transfer Between Storage Locations).

In case of STO from store to DC - fine, i'm able to input  the issuing storage location in me21n.

But in case of STO from DC to store the field EKPO-RESLO (issuing st.loc) is grey and not available for input. Could you help me, why it is grey in such case and is it possible to unlock this field?

Have searched this question through scn, but unfortunately without results.

I'm using SAP IS-Retail, SAP_APPL 617.