Scooby-Doo, Where Are You! (1969)

S1E9 The Backstage Rage

On their way home with pizza, Shaggy stumbles upon a violin case brimming with cash. As he tries to reach the gang to share his discovery, Scooby-Doo rushes off to assist a hurt poodle, leaving the money unguarded. When they regroup, the gang starts to question whether the cash was actually counterfeit.