Why doesn't this store keep two cash registers running at all times? I believe I gave the cashier an extra twenty dollar bill, and the store manager was not able to check the till, because it would halt the store's business.

I am not sure. It is a pretty old business. About the 2nd paragraph, that is just stupid. You should write a review on their website, if they have one, if you haven't already.

The manager is being paid to manage but opted to do otherwise. If only one till was in use, the manager should have switched to the other so that business could continue virtually uninterrupted, and the the till in question could have been counted. If an overage resulted that matched what you thought was the case, the issue would have been resolved. Should only have taken about five minutes.