I did that (almost) once and it worked!

A company gave users Points for doing something on their page and I earned 1800 Points worth about 9 Euro (IIRC)

When I wanted the money they just allowed me to transfer it in 500Point-Steps, leaving 300Points (worth 1.50 Eur).

I saved the source of the "transfer-page", simply added 1800 to the selection, submitted and 2 weks later I had my 9 Euro ;-)

So to everyone dealing with user input: Always check on the server too.