Your agent works for you. If you don't like the deal, then don't sign. If your agent disagrees, tell the agent that you would be happy to cancel the listing and find someone else.
The agent is correct that the lender must approve the short sale. However, sending an unrealistic offer to a lender is a great way to get the lender to ignore you going forward.
If you want to actually make a short sale work, then you need to have the agent produce a "comparative market survey" (aka "comps"), which shows what a "reasonable" fair market price is for the property, set the property to sell for that price, and then find a buyer who will actually qualify for a loan
, if necessary, to make the purchase.
If you do all that, and sign that offer, you'll get the property sold. If you send in a lowball from a buyer with speculative qualifications, the lender may view you as just stalling so that you can live in the property without paying the mortgage, and that will generate a foreclosure
-- sooner than later.
Hope this helps.