The trouble is, we just don't use this sort of formulation in EN (AFAIK); you should try doing a "term search" for simialr expressions like 'bon pour accord' etc. to see how other people suggest handling this.
You could, of course, make something rather longwinded out of it, along the lines of "I hereby confirm this sale" — but I'm not sure that's really very helpful.

HOWEVER, it IS important to make it clear for the person that they must hand write the original phrase in FR, and NOT whatever translation you settle on. So you'll need to make sure you present your "translation" in some way as an "explanation".