If it is a short sale, was it the seller who signed the papers? Then, the bank can counter.
If it is bank-owned and the bank accepted your offer, then I am not sure why would they come back with a counter.
Can you provide more details?
Better yet, consult your realtor.
Lynne gave one interesting reason why this could happen. There are many possible reasons but again the banks do or dont do many things WITHOUT APPARENT REASON!
This is why doing short sales is so much fun. (right---)
best to you
I can't tell you if you can Sue anyone. I suggest you work with your agent and the managing broker to get a full understanding your particular situation, as we here can only speculate.
All the best to you.