A hand shake where an exchange of money (usually a 20$ tip) is being concealed. The person giving the money will hold a bill in their palm. When that person goes to shake hands, the bill moves from one palm to the other in a smooth business transaction.
See the episode of Curb Your Enthusiasm
where Larry David
visits a restaurant and the regular host is accustomed to receiving a 20$ tip via the money shake. Later in the episode, David meets the host on the street and shakes his hand, forgetting about the 20$ bill he was holding. Larry David unintentionally preforms a money shake with the host.