If this is still an issue, you can have the system recalc the quantity on po (same with so) by doing the following:
Off of the library master repair menu, select rebuild application sort files. Enter PO (or SO), select the company code. As some point it should ask you if you want to recalc quantity on po/so. Select yes. That's it. Please note that this process could take a while depending upon how much data you have in the system.. and aborting the process early could cause system-wide problems.. so start it up before you leave for the night. Rebuilding SO/PO sort files usually doesn't take that long though, so I wouldn't be too concerned.. we've only got a handful of clients that push MAS90 with a volume of SO/PO's that I would even think it could be a concern on, so odds are you don't have to worry about it...