There's a couple of steps you can follow to see if you can find an error in your payments if your card amount is incorrect. The easiest place to look through bills is your Sales Log in Reports for the time period you're looking at
If The Salon App says you have more on card than you actually have, go down the list of payments from the day and make sure you haven't selected card instead of cash. If you have, here is how to reprocess that
If The Salon App says you have a different amount of card that you actually have, you might have processed a bill for an incorrect amount. Again go down the list, check each receipt for an incorrect tendered amount and if so you can reprocess using these instructions