The Royals, on Friday, had agreed to a trade with the Oakland A's, sending them pitcher Leo Nunez in exchange for Bradley.
Then, according to a Royals spokesman, the A's learned that Bradley had suffered an oblique injury in his last game for them and the trade was called off.
"Obviously we tried to make the deal, but before the final phase was completed, there was a complication with the medical part and we had to pull out of it," Royals general manager Dayton Moore said.
The A's apparently did not know of the injury when they designated Bradley for assignment on Thursday.
"The A's were not at fault," Moore said.
Bradley, often injured this season, played just 19 games for the A's. He batted .292 (19-for-65) with two home runs and seven RBIs.
Although Bradley has been something of a controversial figure over the years, he had a good working relationship with Royals manager Buddy Bell when both men were in the Cleveland organization, Moore said.
Nunez began the season on the disabled list with a broken right wrist suffered in Spring Training. He's returned to go 1-0 with a 0.87 ERA in six games (20 2/3 innings) for Double-A Wichita.
Although it's unusual to acknowledge a trade that wasn't made, the Royals decided to go public when word of the deal leaked out.
The Athletics have not yet made a statement on the situation.
Dick Kaegel is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.