Gski - classic con. If you don't want to spoil the chance of a sale, let this person know they'll have an exclusive window, of say one week, on whatever it is, and that in that week they can clear up this other person's debt to them as you need the cash etc.
They won't (probably), and if they really want to buy what you're saying, they'll stump up THE CASH. I wouldn't sell something to them against a direct cheque either, but then again I am a suspicious pig.
You'll get screwed otherwise and will lose out in the end.