A Polish discus thrower who won silver at the Olympic games in Rio has donated his medal to pay for the treatment of a three-year-old boy with cancer. Piotr Malachowski's silver medal was purchased by billionaire siblings, Dominika and Sebastian Kulczyk.

The 33-year-old was moved to auction the prize after little Olek Szymanski's mother wrote to the world champion, explaining that her son had been battling retinoblastoma – an eye cancer affecting young children – for two years and needed treatment in New York. The total cost of the surgery is around $126,000 (£95,000).

As he announced the auction, Malachowski posted on Facebook: "If you help me, my silver medal may turn out to be more precious than gold for Olek." He later wrote: "Success," adding that the medal had found buyers. The Olympian was looking to sell the medal for about $84,000 as the other third had already been raised by Polish foundation Siepomaga, ESPN reported.

The auction was closed after Dominika and Sebastian Kulczyk stepped forward. The siblings are worth a combined $3.4bn, according to Forbes.

"We were able to show that together we can do wonders," the Olympic athlete said. "My silver medal today is worth a lot more than a week ago. It is worth the life and health of a small Olek. It is our great shared success."

He added: "Winning an Olympic medal is one of an athlete's life dreams. Of course, the most precious is gold.

"I did everything in my power to get it. Unfortunately this time I did not succeed."

Dominika and Sebastian Kulczyk inherited the fortune from their father, Jan, who died in July 2015 after suffering complications from surgery.