I hope someone can help me. I need a font that looks like the one used for serial numbers on the dollar. I have seen greenback but I need a free font for my art project.

Which dollar? There are so many ...

Make a scan and send your question in the Identification forum : http://www.dafont.com/forum/?f=1

Scanning US currency is not usually a good idea.

Koeiekat, like on the older (90's series) US dollar bill

Like this one

http://www.typography.com/fonts/numbers/inside/greenback

But I need a free version

Scan juste the little area with the font you need.