When setting up a POS for the first time, you need to record the opening cash balance before making your first sale. To do this, make a deposit equal to the amount of change you're starting with, and add a comment such as Opening cash balance or Initial cash balance so you can trace the deposit later.

If you don't empty the register completely every day but only skim off the amount above the opening cash balance, you only need to record the opening cash balance once, since the closing cash balance from the previous day is automatically carried over as the new opening cash balance.

If you want to skim money from your register, you need to make a corresponding withdrawal. Here too, add a clear comment for the cash report so you can trace it later.