Change quantity (MENGE) field in G/L posting

Hi gurus. I need to be able to change the quantity (BSEG/MENGE) field in a G/L document that has already been posted. I found the area in config to add the field to be able to change. So I made the change in config. Then I pulled up a document that already existed, but the field is still grayed out. I also created a new entry, then went back in to change mode, and the field is grayed out. Any ideas?

Thank you

Janet