And you keep assuming that in case of a problem, the best possible solution is to go to the very devil most likely responsible for the additional trouble. That's the thing: there is more then one power- and soul-hungry entity willing to buy things from you. It's not the only game in town at all. The whole incentive assumes you don't care, if the devil in question was the one to spill the beans. Once the secret is out, possible future sales are also out.
Additionaly, I can make an argument that by making the deal short and simple, you are giving the devil free pass to screw you over in whatever way he (or she) sees fit, since there is no contract prohibiting him from doing so. Even more: a devil can screw you over even if you didn't do a deal with him at all, because why not? There is nothing preventing them from being proactive in soul aquisition.
Yes, you'll lose in the end whatever you do - I fully agree with that. It's just that I don't agree with the notion that a short deal would prevent or discourage the devil from actively working to undermine you. They don't need contracts for that.