Could I use the following sentence in a formal letter: "I would very much appreciate your honesty to decline our offer if you were not interested. Please let us know as soon as possible".
I'd just put "Your honesty in regards to the offer would be much appreciated. Please let us know as soon as possible."

It depends who this is to really. I think in any professional matter, honesty should be presumed making such prompts unnecessary. You yourself may end up sounding unprofessional through feeling you need to state it.