Job Demoted Me With 20% Pay Cut

My job demoted me from manager with 20% pay cut because they phrased it protecting the company, they also said i wasn’t doing my job when a lot of the things i implemented as manager, the “new” manager is using. They needed me to get lab certification because i have a chemistry degree and needed someone in the lab with one. My boss told me about this job change and pay cut Friday 12/6/24 , then HR sent over documents to sign Monday 12/9/24. I didn’t sign the new job title and pay but it said went into effect immediately. They tried to give me the same duties as manager with lower pay and title but i disputed it scheduled a meeting with HR and my boss. They just took some responsibilities away to justify the lower salary. This lower pay is stressing me out and i can barely pay my bills now. What can i do? I am looking for a new job and i want to use Fmla but ive been with the company for 7 months now.

Can i sue? It’s in NJ so it’s an “at will” company.