The Bring it On star is spearheading efforts to build a recovery centre for former child soldiers in Uganda, which will help rehabilitate young fighters who were abducted by the African country's rebel army.
Dushku has asked friends and family not to buy her birthday gifts and instead mark her milestone by making donations to the cause.
And Smith has risen to the challenge - appealing to his followers on Twitter.com on Wednesday afternoon (29Dec10), he asked, "Who wants to use money to buy the affections of a girl out of your league? We're auctioning off the next Red State poster, this time so that Eliza Dushku can get her #303030bday wish!"
Many of his online followers bid for the prize, and the winner of the auction offered up $1,000 (£666), which Smith has pledged to donate to Dushku's fund.
Dushku later tweeted an appreciation message to Smith following the prizegiving, writing, "Thank u baby."