Will Ferrell will film the scene with the boy next month

The money will go to the Cancer for College foundation, which is run by one of the comedian's university friends.

The bidder - who has not been named - said the auction meant a lot to him on a "personal basis" as he lost his mother to ovarian cancer.

Ferrell said he was "overwhelmed with the response and generosity".

'College dream'

It was the actor's idea to auction off a non-speaking part to raise cash for the charity.

"This money will help so many young people with their dreams of attending college," he said.

The actor will meet the winner and his son on Friday before they join him on set next month.

The charity was set up by Ferrell's friend Craig Pollard, who survived Hodgkin's disease twice.

The aim was to help provide scholarships to current and former cancer patients.

Around 50 former cancer patients have benefited from the $200,000 (�99,000) that has been raised by the organisation since 1993.

Step Brothers, due for release next year, stars Ferrell and Adam McKay as two spoiled sons who become competitive stepbrothers when their single parents marry each other.