Looking for help on a foreign currency spreadsheet. A bit complex

In reference to the attachment:

I'm working on a foreign currency spreadsheet for my work. It needs to store the amount of each denomination of each currency so we can keep track of it. I've got the formulas working for that actual exchange, no big deal, but I'm trying to find the best way to automate the recordkeeping of the denominations.

What I'm trying to do is this: on the tab 'Selling Foreign Currency', since we are getting rid of the currency, if sell I4 reads MXN, it should SUBTRACT the denominations (maybe after pressing a button...we could tie the macro to it) from the Peso tab. And same for all the other denominations (euro, pound, franc etc). Same for the 'Buying Foreign Currency' tab, except adding.

I realize this may be a lot of work, if its even possible, but if someone could help even with a part of it I could probably figure out the rest

Thank you!

Foreign Currency Test.xlsm

Thank you for your reply! Can I get a little help on how to use that variable to subtract the denominations from the columns on 'Selling Foreign Currency' to the desired sheet(Peso)? Not really sure how to start that function.