“Money in the bank” (often given as “like putting money in the bank” or “like money in the bank” or “like money in a bank”) means a sure thing, a guaranteed success, a safe investment. “Money in the bank” has not always been safe, but the saying has been used since at least the 1880s.
   
Buying a diamond is “like putting money in the bank.” Buying a piece of real estate is “like money in the bank.” Buying good clothing is “like money in the bank.” Basketball player Larry Bird’s jumpshot is “like money in the bank” when he shoots it.  The phrase has been very popular and is often shortened to simply “money.”
 
 
(Oxford English Dictionary) 
money, n. (and adj.)
Chiefly U.S. colloq.(to be) money in the bank: (to be) a guaranteed success, a reliable good performer, esp. in sport or the entertainment industry.
1939 Zanesville (Ohio) Times Recorder 3 Jan. 11/2 Money in the bank, dearie, money in the bank. That’s what diamonds are.
1948 P. MARTIN Hollywood without Make-up 59 Whatever a producer has to pay, he is lucky if he can lure Lillian Hellman, Charley MacArthur or Robert E. Sherwood out from New York. They are all money in the bank.
1990 D. DIMAGGIO & B. GILBERT Real Grass, Real Heroes xiii. 175 In 1941 Mickey Owen was money in the bank behind the plate as the Dodgers took the field to try to square up the Series.
1999 Sports Illustr. (Electronic ed.) 4 Oct., Since rejoining the fin de siecle Fins, Martin has been money in the bank, averaging a team-high 22.2 yards per catch.
 
13 January 1885, Atlanta (GA) Constitution, pg. 14, col. 3:
Nothing that Atlanta has done advertises her so well or stands her in such good stead as her admirable streets. (...) Putting money into our streets under the present contract is like putting money in a bank. 
 
21 May 1885, Decatur (IL) Daily Republican, pg. 3, col. 2:
IT is like putting money in the bank to invest in one of those Everett or Haines Bros. pianos sold by C. B. Prescott in Opera block. See either instrument and you will buy.
 
9 June 1889, Themis (CA), pg. 8:
Boffkins says his wife’s bustle is like money in the bank—it’s something to fall back upon.
   
22 July 1897, Philadelphia (PA) Inquirer, pg. 3 ad:
Better buy now, before the new tariff puts up the price of good mattings—it’s like money in the bank for you.
(Summer mattings at Apt. Bros.—ed.)
 
14 August 1897, Frederick (MD) News, pg. 5, col. 6 ad:
It’s almost like money in the bank for you.
The New Carpet and Wall Paper House.
L. E. MULLINIX
(Mosquito proof canopies—ed.)
 
19 July 1899, Duluth (MN) News-Tribune, pg. 1:
LIKE MONEY IN THE BANK
CORN PROSPECT EXCEEDS ANY-
THING EVER YET KNOWN.
   
4 February 1901, Dubuque (Iowa) Daily Telegraph, pg. 2, col. 4 ad:
Like Putting Money in the Bank To Buy Shoes Now.
(Levi’s Shoe Dept.—ed.) 
 
28 September 1902, New York (NY) Times, pg. 20, col. 4 ad:
BOROUGH PARK fits particularly well the case of the young man who aims to own his home before old age impairs his earning capacity.
 
Every dollar invested in a home or lots here is like money in the bank.
(Borough Park Co.—ed.)
 
29 September 1905, New York (NY) Times, pg. 10, col. 5 ad:
When a man saves a big slice of his clothing expenses by deserting his tailor and coming to us, he often feels justified in blowing himself to some extra luxury.
 
Like a suit with luxurious silk lining, which of course won’t wear as well as plain serge, but which looks and feels like money in the bank.
(Rogers, Peet & Company—ed.)
     
8 December 1906, Fairbanks (AK) Evening News, pg. 8, col. 1 ad:
Owning a diamond is like money in the bank.
(“Suter. The Live Jeweler”—ed.) 
 
13 March 1908, Oakland (CA)

, pg. 5 ad:
Assets in pianos purchased at factory cost, like money in the bank, always available.
 
31 October 1919, Racine (WI) Journal-News, pg. 24, col. 2 ad:
If you own property in RACINE it is like money in the bank.
(Asylum Ave. Tract—ed.)
     
15 October 1937, Chicago (IL) Daily Tribune, pg. 23:
Dough in the Ice Box Is Like Money in the Bank
See the Saving! 
 
8 September 1940, New York (NY) Herald Tribune, This Week magazine, pg. 2, col. 1:
LIKE MONEY IN THE BANK
By Clara Belle Thompson and Maraget Lukes WIse
(...)
“And as a plain business proposition I’d rather lease it to you than to anybody I know. You may not realize it, Bill Hagans, but your reputation is just like money in the bank—and plenty of it!”