American comedican Henny Youngman (1906-1998) frequently told of a doctor who gave a man only six months to live. The man couldn’t pay his medical bills, so the doctor gave him another six months!
 
Youngman has been cited in print with this joke by at least 1974. However, the joke has been cited in print since at least 1966; in 1973, the joke was credited to American actor Phil Foster (1913-1985).
 
   
Wikipedia: Henny Youngman
Henry “Henny” Youngman (original Yiddish surname Yungman; 16 March 1906 – 24 February 1998) was a British-born American comedian and violinist famous for his mastery of the “one-liner”. His best known one-liner was “Take my wife…please”.
 
In a time when many comedians told elaborate anecdotes, Youngman’s routine consisted of telling simple one-liner jokes, occasionally with interludes of violin playing. These depicted simple, cartoon-like situations, eliminating lengthy build-ups and going straight to the punch line. He was known as “The King of the One Liners”, a title conferred to him by columnist Walter Winchell. A stage performance by Youngman lasted only fifteen to twenty minutes but contained dozens of jokes in rapid-fire succession.
 
23 March 1966, Circleville (OH) Herald, “Try and Stop Me” by Bennett Cerf, pg. 4, col. 3:
There’s a fellow leading a normal life in Amarillo, Texas—and active as a bird dog—whose doctor once gave him only six months to live. When the six months had elapsed, however, he hadn’t paid one penny of his bill—so the doctor gave him another six months.
 
1 July 1973, Aberdeen (SD) American News, “Earl Wilson’s New York,” pg. 6, col. 5:
PHIL FOSTER told of a friend whose doctor gave him six months to live. And when the guy didn’t pay his bill ,the doctor gave him another six months.
 
16 April 1974, Lima (OH) News, “Hollywood Hotline: Youngman Calls With 1-Liners” by Jim Bacon (NANA), pg. A6, col. 7:
“Guy goes to a doctor who only gives him six months to live. The guy can’t pay his bill so the doctor gives him another six months.”
   
Google Books
The Reader’s Digest
Volume 108
1976
Pg. 154:
Henny Youngman says he knows a doctor who gave a guy six months to live. The guy didn’t pay his bill, so the doctor gave him another six months.
— Bob Goddard in St. Louis Globe-Democrat
 
Google News Archive
19 February 1977, Daytona Beach (FL) Morning Journal, “After Nearly 50 Years In Show Business: Observations From Henny Youngman” by Jim Willse (AP), pg. 13C, col. 2:
“The docor gave me six months to live. I couldn’t pay my bill by then, so he gave me another six months.”
   
Google Books
How to Live to be 100—or More:
The Ultimate Diet, Sex, and Exercise Book
By George Burns
New York, NY: Putnam
1983
Pg. 143:
There were no health plans in those days, and even $Z.00 for a house call was rough. But Dr. Stern was very considerate. He gave Mr. Klein six months to live, and when he found out he couldn’t pay his bill, he gave him another six months.
 
Google Books
Laugh Off:
The Comedy Showdown Between Real Life and the Pros
By Bob Fenster
Kansas City, MO: Andrews McMeel Universal Company
2005
Pg. 148:
Comic Henny Youngman: “A wonderful doctor gave this guy six months to live. When he couldn’t pay his bills, he gave him another six months.”