Boy, did he make good on that contract.
He hit 39 home runs and tallied 114 RBIs that year, finishing fourth in AL MVP voting, before departing for a tour with the Blue Jays. Toronto released him in April 2008, and a 39-year-old Thomas knew exactly how he wanted to keep his illustrious career alive.
Within days, he returned to Oakland. That's where he would last put on a Major League uniform.
"I never wanted to leave here," Thomas said at the time. "This was my first choice, to come back here."
There's no doubt Thomas will be remembered most for a 16-year run with the White Sox, but in short time he left his mark in Oakland, where A's fans joined in celebrating the Big Hurt's election into the Hall of Fame on Wednesday.
In his first time on the ballot, Thomas received 83.7 percent of the Baseball Writers' Association of America vote, well above the needed 75 percent total for election.
"I'm just so excited that I'm in the Hall of Fame," Thomas said on MLB Network, shortly after hearing of his election. "You can only dream so big, and this is just amazing.
"The game has meant so much to my family and I my whole life. Today I'm just honored. I'm so short of words right now. It's hard to think. ... I'm blessed, and I'm just so happy for all of the writers who voted me in because I gave everything I had and more."
The first baseman/designated hitter joins Greg Maddux and Tom Glavine, along with retired managers Tony La Russa, Bobby Cox and Joe Torre, as part of a revered 2014 Hall class to be inducted on the weekend of July 26-27 in Cooperstown, N.Y.
He's the first Hall of Famer to play the majority of his games at DH (56 percent).
"We congratulate Frank on joining the immortals of baseball," the A's said in a statement. "While he only spent two seasons with our organization, we had the distinct privilege to experience his greatness as a player and as a person."
Thomas finished his career with 521 homers, tying him for 18th with Ted Williams and Willie McCovey on baseball's home run list, along with a .301 average and 1,704 RBIs. The five-time All-Star turned in a .419 on-base percentage and .555 slugging percentage, taking home AL MVP honors in 1993 and '94 and Louisville Silver Slugger honors in '91, '93, '94 and 2000. He also earned the '97 batting title with a .347 average.
The 6-foot-5 slugger topped the .300 mark in 10 seasons and batted .330 or higher four times. Moreover, Thomas produced seven straight seasons of hitting .300 with at least 20 homers, 100 walks, 100 RBIs, 100 runs scored, a .400 on-base percentage and a .500 slugging percentage from 1991-97.
These credentials make him one of the best hitters in the history of the game. And in an era clouded by performance-enhancing drugs, it was Thomas who disregarded the silence and remained outspoken against their use.
"Beyond his special talents, Frank Thomas was the consummate professional who respected the game, his teammates and his opponents," the A's statement read, "and he exhibited the kind of class every player should aspire to. He is richly deserving of this honor."