What can I do if employer threatens to withhold pay if contract is not signed with their stipulation

I have worked for a Private Higher Education institution for 13 years. I meet all the requirements necessary for my job description. When hired for this job I was not required a PhD. I'm currently working on my PhD. My employer wants to add a stipulation to my contract that states that they will not renew my contracts in the future if I don't complete my PhD by a certain date.

I expect to finish my degree, but feel that agreeing to the stipulation is prejudicial to my interest. They are threatening to withhold paycheck and benefits if I don't signed the contract as stated by them. I had signed the contract with a letter expressing acknowledgement of their intent, not agreement, but in no way giving up my rights to the appeal process of my organization nor legal rights. They are threatening to withhold my salary and benefits if I don’t remove the letter from the contract. There are other professors who do not have a PhD, nor are seeking a PhD who are not been asked to do the same.

The last day of my last year contract was June 30. I’m out of contract, but they are expecting me to singed under their conditions so there is no interruption in payroll.

What are my options in response to their threat to withhold my paycheck and benefits until I signed their contract under their terms?