My question involves labor and employment law for the state of: MN

Can a catering service keep money that is designated as a gratuity on an invoice? A restaurant has a catering department where the servers are not being tipped at the end of an event even when there is a gratuity listed on the invoice that is presented to the customer. The customer thinks that a gratuity is already added to their bill so they don't usually tip in cash but the restaurant is keeping the gratuity. The servers are paid more than minimum wage ($9.00-10/hour) and have been told that the tips cover the increased wages. There isn't anything listed on pay stubs as 'tips' and servers are not claiming anything in tips, even if they are tipped a little more in cash by the customer.

Would the situation be different if there is an 18.5% 'service charge' added to the invoice but no gratuity specified? If the customer does leave an additional tip with the invoice, it is not distributed to the servers, so the catering service keeps the 18.5% and 15% on top of the bill.