My attorney wants to void the terms of our written agreement, is this legal? Ethical

I was involved in an auto accident with injuries(mostly soft tissue) in 08. After almost 2 years the insurance company offered a settlement of $30,000, I asked for $35,000. They refused and told me to get an attorney. Found an attorney who wrote in our agreement that he would only collect his 40% on any amount over the insurance company''s offer of $30,000 this was signed in 2010. Now the attorney said the case was not as presented to him and they will not go above the $30,000. He feels that if we go to court we would not be awarded any more money and there is a chance it could be less. . He did say juries don''t like "soft tissue" injury cases. He also said if I accept the settlement that he will take $5,000 as his fee + 1700. for costs. And he wants me to sign a settlement agreement to that effect which also states the original agreement is null and void and that I agree to the $5,000 and state that my attorney has benefited me financially.