"Please give me a dime (or another amount) for a sandwich” is what a beggar might request. The line usually means that the beggar is hungry and needs the money to buy himself a sandwich to eat. A joke ending has a person taking this as a transaction request and responding, “Let me see the sandwich first.”
The joke has been cited in print since at least 1929 and was popular during the 1930s economic depression.
Volume 123, Issues 1-9
Beggar, to Sandy McTavish: “Wud ye please give me a dime for a sandwich?”
Sandy: “Show me the sandwich.”
5 October 1934, Thomasville (GA) Times-Enterprise, pg. 7, col. 2:
Beggar:—Will you please give me a dime for a sandwich?
B. V. Sober:—Let me see the sandwich.
Norfolk and Western Magazine
W. H. “Hub” Beckner said that Taylor Austin went up town a few days ago for lunch, and just as he walked in the restaurant door, a fellow walked up to him and said, “Buddy, how about a dime for a sandwich,” and Taylor said, “Let me see the sandwich first.”
17 January 1940, Charleston (WV) Daily Mail, “Hardly a...Column” by The Stroller (R. W. Jackson), pg. 7, col. 4:
AN EMPLOYEE at Woodrums’ store tells this one on Mrs. Dorothy Koester, switchboard operator at the store.
Tuesday morning as Mrs. Koester headed for work she was stopped on the street by a man who asked:
“Lady, will you give me 10 cents or a sandwich?”
Although Mrs. Koester knew what the man meant, she pretended as though she thought the man was trying to sell her something to eat. She told the beggar:
“Let me see the sandwich fist.”
6 November 1954, Chicago (IL) Daily Tribune, pg. 15:
Hobo: “Sir, will you give me a dime for a sandwich?”
Gent: “Let me see the sandwich.”
10 April 1957, The Prospector (El Paso, TX), “On Campus” with Max Shulman, pg. 2, col. 3:
DR. MAGRUDER: Howdy, folks. A funny thing happened to me on the way to my doctorate. A mendicant approached me and said, “Excuse me, sir, will you give me 25 cents for a sandwich?” I replied, “Perhaps I will, my good man. Let me see the sandwich.”
yes, it is a known joke but to some it happens in real life. I have even encountered this situation on the streets.