That said -- and this is not legal advice specific to your situation, and I am not your attorney -- its not clear whether or not you have a fully signed contract in place (read your last sentence). Unless there has been mutual acceptance of a contract, there is no contract, although our case is complicated by the auction bid. Bottom line: if there's a few thousand dollars at issue, it merits consulting an attorney. And rely on the free advice you get here at your peril.
However I had an instance with Freddie Mac about 4 years ago where they did almost the same thing. They sent the contract over for for $10k less than we had last countered. We weren't sure if they made a mistake or if they just realized that our original offer was in line or if there was something wrong we didn't know about! The listing agent wouldn't say. Of course I told my buyer about the potential risk but my buyer still signed immediately. I called the escrow company that FM regularly used and asked if they had seen this before and they hadn't seen FM retract any offers. Later on in the transaction they wanted to recall it. My buyer consulted an atty and I THINK they said they could get the sellers for specific performance if they backed out. Don't quote me on that. Eventually FM stayed put and we closed.
Confer with an attorney if you don't have an agent
Lynn911 Dallas Realtor & Consultant, Loan Officer, Credit Repair Advisor
The Michael Group - Dallas Business Journal Top Ranked Realtors