[OT] Can I make a prototype Free and sell the final product?

[William Ballard <nospam_40811@alltel.net>]

I'm hacking together a little prototype for somebody, mostly as
a curiosity and kindness.  It'll be just a prototype.
If they like it, I'll flesh it out and charge them money.

Regardless, I'd like to put the prototype on SourceForge.
(It might help close a sale.)

Do I need some kind of dual license?