@villagepubjason There is no such setting currently in Square. But, it seems to me that this is a simple as having your bartenders just ignore the cash sales line and the cash owed line since the cash for this is already in the register. All they should need to do is take out the non-cash tip amount they are owed and everything should balance perfectly, assuming that the bartenders always transact case sales from the register. That's the way that I see other local restaurant owners do this that use Square, at least.
That being said, if you feel very strongly about this, you should head over to the feature request board and submit it as a request for the engineers to consider.
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