What should i do to get the compensation we have agreed with my employer when i signed my contract?

Hi just need some advice. My husband is on a working visa H1b and on a 3 yr contract with his employer. Since wanted to stay here for good he decided to sign another 2 yr contract with his employer in exchange for an immigrant petition which is alredy been filed and on process and With a binding agreement that after signing up extension each year he will be compensated additional $1/hr to his current rate until he finishes the total 5 yr contract. He just had his 2nd yr last august and entitled with $1/hr increased i supposed. He has unused PTO so as their policy they give it cash, and he's supposed to have it by 2nd week of august after his 2nd yr anniversary but suddenly his employer informed him that they noticed that the rate they are paying him is a mistake that the rate is higher than the current LCA. Since it was their mistake paying him that rate for more than 1 yr they are not requiring him to pay back but they are suggesting to pay him according to the LCA which is $3 lower than the current rate. Therefore the supposed increased will be off set. So my husband wouldnt agree with the suggestion and wishes to retract the employment extension since the agreed salary increased cannot be fulfilled which is one of the reason he decided to extend his contract with them. Up to now we are still waiting for his PTO, and they say they are still discussing about it. They are still paying me with tha same rate which they claim to be a mistake up to now without the supposed increased. What should i do? Please help me.