I agree with the different replies to your post.
First off, especially in this market, I don't advise making an offer and entering into a contract on a house when your own is not even in contract unless you're a) financially able to carry two mortgages and b) able to qualify for carrying two mortgages.
Next, to your real question: The audacity of this RE agent is mindboggling!
He's basically asking you to become an accomplice in defrauding the lender! In a short sale, the lender agrees to accept less money than is owed, with the clear understanding that the borrower/seller receives no monies whatsoever as a result of the transaction.
What is happening here is both ethically and legally wrong and I agree with another poster - this agent needs to be reported, not only to the broker but to the local licensing agency, possibly even NAR, if he is a member (a "Realtor"). This kind of behavior is part of what gives the rest of us a bad name!