You’re Missing the Point (1940)

Original title: Ahí está el detalle

Cantinflas, the boyfriend of the servant of a rich industrial man, gets into the house in order to kill a mad dog. Suddenly this man appears so the servant tells him that Cantinflas is his wife's brother (Leonardo), who had been lost for years. The rich man then remembers that his father in law's testament could only be paid when all brothers get together, so treats Cantinflas, a real bum, as a king.