I've never heard "pro forma" desrcibed as pre-payment.
Hi Tom, I did an "Ask Jeeves" and found this for "pro forma":
1 : made or carried out in a perfunctory manner or as a formality
2 : provided in advance to prescribe form or describe items <pro
I guess "provided in advanced" has come to apply not just to the
invoice but to payment as well, at least in the world of wholesale.