How do I create a form to reduce a quantity as goods are booked out?

I need to create a form which will be an Advice Note. This is meant to bring up a job with item lines and book out a quantity from which ever line item I select. It then needs to reduce that line item quantity in the original table and print out an advice note reflecting the quantity I have booked out. Can anyone advise me as to how best to do this..I need a field that holds a quantity in memory while the transaction is done and then release that value for the next time.