How do I capture the value of the AdterUpdate event to prevent Debug Error

On this  program I have been working on I want to catch an error before it starts. I think I can do this if I can find out if a FIELD has been updated/changed.
 
The cashier scans in an item but the customer decides they don’t want the item or the cashier screwed up so they want to delete the item. If you RIGHT CLICK the record you can choose to delete the record.  However,IF you try to delete the record BEFORE you have changed the quantity an error occurs and you have to debug.  I am trying to prevent this error.
 
Here is the how process currently works:
1.Cashier scans the item and realizes it’s a mistake
2.Cashier MUST change the quantity field BEFORE deleting the item
3.Cashier Right Click record now and chooses to delete and it works OK.
               a.If cashier forgets to change the quantity BEFORE deleting the error occurs

How can I check to see if the quantity has been changed in the QUANTITY field when the cashier RIGHT CLICKs the record to delete