It might be too late, but here is what I learned:
get the money up front (don't spend it!)
in art school (in germany) you price your paintings like follows:
(lenght + width (metric system)) x factor (depends on your experience level, sales, gallery representation)
so you have a 100cmx100cm painting and are still learning:
(100+100)x2= 400€
If he doesn't like it, give him some money back, but keep your material costs and whatever you charge for the time you spent on it.
Tell him so beforehand, though :)
