The 33-year-old world champion wrote on his Facebook page that he was moved to action his prize after receiving a letter from the mother of a boy called Olek who said he had been battling eye cancer for two years and that treatment in New York was his only hope.
"I fought for gold in Rio. Today I'm calling on everyone to fight for something even more precious," Malachowski wrote on Friday to announce the auction.
"Success," he later wrote, saying the medal had found takers.