Dear Shmuel---I do understand your problem. I really do, and I only wish I had an easy and workable answer for you. I know how frustrating it must be for you as you are laboring to do your good work, only to be informed that what you want to do is basically unsound, unrealistic, and in many quarters, considered to be unethical.
But, regardless how you put it, as you have responded today, you would still be asking a professional grant writer to work on a contingent-pay basis---that is, you are telling a prospective grant writer, "You do the work, estimate your hourly fee, and we'll submit the grant proposal. And then we will pay you, "perhaps," if we get the money."
The salary burden might be lifted for the moment for you, but should the grant proposal be denied, you lose---and the grant writer certainly loses.
I'm not talking to you about your proposed agreement in the context of ethics or that I truly don't believe that you will find a competent and experience professional to work on the basis you wish---instead I was hoping to convince you about the real and serious consequences possible for the grant writer and for your orgnization. I told you that I had listed them (and they are real) in my article. One of them, perhaps the most usual, pitfalls regarding contingent-pay is:
"Grant-writing expenses are seen as part of an organizations operating budget. Few if any foundations, corporations, or governmental organizations are willing to make a grant when a portion of the money granted is to be used to pay a grantwriting fee. Remember, the grant is being requested for a specific project, not to offset operating expenses nor to disguise a professional fee. A non-profit or a grant writer that fails to take the possibility of such a caveat into consideration may be facing a rude awakening."
Of the eleven consequences I have come to learn regarding contingent-pay schemes, the one above is perhaps the one which trips up organizations the most.
Not only is what you propose still a contingent-pay arrangement, but I cannot believe that you will find a true professional willing to work on a "perhaps" pay basis. Ask, and you might find someone who will simply volunteer their services. The Addendum in my article provides some leads to grant writing professionals for their usual paid engagements, but it never hurts to ask for some pro bono work. We have all done so from time to time.