Mark Ballas Gives the Shirt Off His Back ... for Charity

"Dancing with the Stars" stud Mark Ballas single-handedly raised more than $12 THOUSAND for the elderly last weekend -- all because he forgot to bring a change of clothes to a charity benefit.

Mark volunteered to perform at the 4th Annual Avamar Foundation benefit in Warren, Ohio -- representing The Beat of Ohio dance studio -- but we're told, he only brought one outfit ... and after performing ... had to borrow a shirt from the host.

Sources at the event tell TMZ Mark was so moved by the benefit, he decided to sign the shirt ... and auction it off. He also threw in front row VIP tickets to 'DWTS' for two nights, backstage passes, and pictures with the other dancing pros.