“Bankers do it with interest”

“Bankers do it with interest” is one of many professional “do it” jokes that have been put on T-shirts and bumper stickers. “Bankers Do It With Interest” has been cited in print since at least July 1978.
 
     
CafePress
“Bankers Do It” Gifts
 
Google Books
July 1978, Texas Monthly, pg. 181 ad:
Bankers Do It With Interest
(“DO IT” T-shirts—ed.)
 
15 April 1979, Los Angeles (CA) Times, pg. OC-B1:
Consider the simple “do it” theme that is proclaimed from every other license plate frame: Bankers do it with interest, divers do it deeper, etc. etc.
       
Google Books
How do they do it?:
A collection of wordplays revealing the sexual proclivities of man and beast
By Reinhold Aman
Waukesha, WI: Maledicta Press
1983
Pg. 11:
Bankers do it for money.
Bankers do it interestingly.
Bankers do it only with collateral.
Bankers do it with interest.
 
Google Books
Buck Fever: A Blanco County, Texas, Novel
By Ben Rehder
New York, NY: St. Martin’s
2003, ©2002
Pg. 142:
It was green, the color of money, and it had a bumper sticker on the rear window that said, BANKERS DO IT WITH INTEREST.
 
New York (NY) Post
RETIRE TO THE BEDROOM
SEX FEST AT OLD-TIMERS’ HOTTEST SPOT
By STEFANIE COHEN
Posted: 3:09 AM, January 25, 2009
(...)
Tucker sports a gold charm around his neck that reads, “Bankers do it with interest.”