Sublease

4 years ago, we sole a beauty salon but my name is still on a lease agreement (as sublessor) until Sep 2010. We called the buyer as A. A later sole to B without noticing me. The landlord was aware of this transfer as well. In 2008, B failed to pay the rent and the landlord request me & buyer A to pay the balance of $10K in order to take the salon back. Buyer A verbally agreed to pay but didn't pay and now the balance is up to $20K. The landlord will proceed the lawsuit against me & buyer A if we don't settle by 8/31/09. Am I or is A responsible for the liability? The landlord failed to inform me the lease transfer between A to B, can we use this as excuse not to pay? Am I responsible for the time that A made money without paying the rent? What should we do in this case? Is it worth to hire lawyer or pay $20K? Thanks.