Short sell problems -tax

Bank approved the short sell at certain price, when the house was on market, we offered 10K more than the approved price.   Now bank wanted us to pay for the back tax.  The seller has not paid tax for 6 years, and bank will only pay for two years.  So we need to pay the 4 year tax - 20K.

How can we counter offer?  How can the bank forget initially about the tax?

Any Suggestions?