老婆饼 Wife cafe (a joke)

I am telling you a joke, I read the original one in Chinese. I try to translate it in English. It won't be interesting if it is translated in French since there is no such a ambiguity in French.
A customer bought a beef bun at a food stall. He came back after a while, a half-eaten bun in his hand. "Hello, owner, why there is no beef in this bun? Your product label says that this is a beef bun." The owner realized that he forgot to add the beef in the fillings of the buns this morning.  But he won't recognize his mistake since he has to refund the customer. He responded instead, "Who says that a beef bun must have beef inside? Look, this is a pineapple bun I bought this morning at a pastry shop. There is no single trace of pineapple in it. Also, I bought a wife cake. How do you expect to find a wife in a cake?"