Displaying and computing row values differently from underlying sheet

I have a simple AppSheet app that allows adding manual transactions to a personal finance spreadsheet populated by Tiller.

Most of the transactions have negative values in the Amount column, representing purchases.

Since the common case for new entries is that they are purchases, I’d like to be able to enter the amount as a positive value, with a checkbox “deposit”.

Any suggestions for how to go about this? I played around with virtual columns and with setting the formula for the real Amount column but didn’t get anywhere. A complicating factor is that both Tiller and the App Sheet app will be adding rows to the underlying sheet.