It may take some legwork, but if possible write up something with the fact your receiving the 600 no matter if he buys the bike or not and you sign it and he sign it. Save a copy. You will have to no doubt scan it, email him, and he will have to print, sign, scan, send it back to you. You could also ask him to notarize it, so you simply notarize it, and he should be able to have any commissioned officer notarize his sig as well.
It's legwork, but it will cover you from any BS should you choose to be worried.
The guy who wants to buy my car is also deployed, but we've talked on the phone, hahah.