The Twitter.com post, sent via @RealFerrellWill on Saturday (24Mar12) suggested the writer would offer up $1 (62 pence) for every retweet of the message "R.I.P. Trayvon Martin".
But the generosity did not belong to the real Will Ferrell.
The Old School star's publicist says, "That is definitely NOT Will Ferrell who does NOT have a Twitter account."
Hundreds of followers, including celebrities, fell for the hoax - and retweeted the post, hoping it would benefit the Trayvon Martin Foundation, which does not exist.
Martin was shot dead last month (Feb12) by a Neighbourhood Watch volunteer, who thought the Florida teenager was acting suspiciously as he walked through a gated community. The teen was reportedly unarmed.