Is it legal for a company to have their employee break change for a customer's bill using their own tip money?

My friend was explaining to me how when they waitress they have to use their own tip money to break change for a customer's bill, but they keep track of the tip money spent and the company supposively pays them back at the end of their shift of the tip money they had to use. I've never heard of this, and I don't know if that's legal.