As a developer, I've been on the receiving end of "we promised the client we could do this, and they've already paid us, so get to it" more times than I can count... it's my least favorite salesman trait, by far.
To you, it is completely irrelevant whether the reason your employer ordered you to trash your life was because the salesman sold vaporware, or because of the phase of the moon; the outcome for you is the same regardless of the reason the order was given. You have to change your policy of obeying such orders.
My least favorite is salespeople who give product away for free (including the support thereof), hurting the bottom line, since their commission is on gross.