“A deal is a deal” means that a deal (such as a contract) has been agreed to and that its conditions shouldn’t be changed. “None o’ yer backin’ out; a bargain’s a bargain, a deal’s a deal” was cited in print in 1874.  “A deal’s a deal” was cited in print in a business magazine in 1907. “A deal is a deal and a bargain is a bargain” was cited in 1914.
 
The phrase is especially popular in business.
 
 
Google Books
Eulalie, or, The Red and White Roses
By William Stephens Hayward
London: H. Lea
1874
Pg. 416:
“None o’ yer backin’ out; a bargain’s a bargain, a deal’s a deal. I’ve sold you the gal, and I mean to have the gold. If you don’t like it, go to blazes! You’ll have none at all!”
 
Google Books
Waitaruna:
A Story of New Zealand Life
By Alexander Bathgate
London: Sampson Low, Marston, Searle, & Rivington
1881
Pg. 136:
“A deal’s a deal,” replied the man of iron.
 
Google Books
Mrs.Tregaskiss:
A Novel of Anglo-Australian Life
By Mrs. Campbell Praed
New York, NY: D. Appleton and Company
1895
Pg. 400:
“A gift’s a gift, and a deal’s a deal.”
   
Google Books
June 1907, The Business Man’s Magazine, “John Hake, Business Gambler” by B. P. Owie, pg. 28, col. 1:
“Wonder ‘f he wants t’ back out the timber deal,” asked one.
 
“Not with me,” said another. “Tax collector’s got mine, an’ b’sides a deal’s a deal.”
   
9 December 1911, Manitoba Free Press (Winnipeg, Manitoba), “Good Stories of the Business World: The Business Gambler” by B. P. Owie, Magazine Section, pg. 9, col. 3:
“Wonder ‘f he wants t’ back out the timber deal,” asked one.
 
“Not with me,” said another. “Tax collector’s got mine, an’ b’sides a deal’s a deal.”
 
26 March 1914, Duluth (MN) News Tribune, “Uncle Sam Should Not Squeal,” pg. 8, col. 1:
However this may be, a deal is a deal and a bargain is a bargain. Uncle Sam should not “squeal.”
 
5 March 1921, Philadelphia (PA) Inquirer, “Women to Take Gas Fight into Politics,” pg. 12, col. 7:
“I believe that a deal is a deal, and should be lived up to,” was the response.
 
11 July 1926, The Sunday Courier and Journal (Evansville, IL), “Then Came the Dawn” by Jack Lait, Magazine Section, pg. 5, col. 3:
Did Carl then, like a good sport, say “a deal is a deal, dearie,” and choose one of her propositions?
 
21 July 1928, Oakland (CA) Tribune, “Bandit Land” by Owen Lattimore, Magazine Section, pg. 4, col. 1:
The Elder Brethren are a businesslike outfit, and a deal is a deal.
 
18 April 1932, The Sunday Star (Washington, DC),  “Mutt and Jeff” comic strip by Bud Fisher, pg. ? (comics):
A Deal Is A Deal
 
2 February 1937, San Francisco (CA) Chronicle, “Moon Mullins” comic strip by Willard, pg. 6-H, col. 1:
GO ON AND TAKE IT! A DEAL’S A DEAL
 
Google Books
Hardy Boys 23:
The Melted Coins
By Franklin W. Dixon
New York, NY: Grosset & Dunlap, Inc.
1944 (2008 printing)
Pg. ?:
“I understand you made a deal. And a deal is a deal!”
 
Google Books
Blood, Sweat, and Stanley Poole:
A Comedy in Three Acts
By James Goldman and William Goldman
New York, NY: Dramatists Play Service
1962
Pg. 7:
POOLE: Listen, I said I’d pay. I’ll pay. A deal’s a deal. But I can’t do it now.
   
Google Books
Crystal Star
By Albert A. M. Stella
CreateSpace
1978
Pg. ?:
Joachim: Fritz, a deal is a deal, right?
 
OCLC WorldCat record
A deal is a deal, or, How much for your head and other stories : a reader for Burkina Faso secondary schools
Author: R Mathieu Ouedraogo
Publisher: [Burkina Faso : s.n., 1989]
Edition/Format:   Book : Fiction : English
 
Google Books
The Haggler’s Handbook:
One Hour to Negotiating Power
By Leonard Koren
New York, NY: W.W. Norton
1992, ©1991
Pg. 122:
It may be best to simply walk out and refuse to come back until he agrees to adhere to principles — a deal is a deal.
 
OCLC WorldCat record
A Deal Is a Deal: Antenuptial Agreements regarding the Religious Upbringing of Children Should Be Enforceable
Author: Jocelyn E Strauber
Edition/Format: Article Article : English
Publication: Duke Law Journal, v47 n5 (19980301): 971-1012
Database: JSTOR Arts & Sciences IV Collection
 
Google Books
Understanding American and German Business Cultures:
A Manager’s Guide to the Cultural Context in which American and German Companies Operate
By Patrick L. Schmidt
Montreal: Meridian World Press
2001
Pg. 49:
The phrase “A deal is a deal” is often heard, meaning the contract can’t be changed or broken.
 
OCLC WorldCat record
OPINIONS - FRANK HAYES reminds us of the cardinal rule of business: A deal’s a deal, no matter what.
Author: Frank Hayes
Publisher: [Framingham, Mass., etc., International Data Group, etc.]
Edition/Format: Article Article : English
Publication: Computerworld. 35, no. 9, (2001): 72
Database: ArticleFirst
     
English Forums
Deal is deal?
Approved answer (verified by Mister Micawber)
5th August 2011
I’ve always heard “a deal is a deal”, not “deal is deal”.
 
Basically, an agreement is an agreement.
 
Typically, if you made an agreement with someone and they are backing out/canceling it.
 
You are, simultaneously, stressing that you had an agreed upon deal and are slightly displeased with the current situation.
 
Birth That Book by Kelly Epperson
January 5, 2013
A deal is a deal is a deal. Deal?
A deal is a deal. We’ve all heard that phrase. It means essentially that if you agree to something you follow through on your promise. You do what you said you would do and the other person does the same. “We made a deal and we stick to it.”