Boss is withholding salary increase he agreed to in writing

I have been at my current job as an insurance account manager for just over 3 years and have been unhappy for most of that time. In November 2019, I resigned from my job and accepted a position at another company in the same industry. My boss did not really accept my resignation and asked why I was leaving. My reason (besides the fact that he is an impossible person to work for) was also that the small office I work in was a toxic environment and that my coworkers, including my manager, did not pull their weight, too much was expected of me for my position and everything about the job was unprofessional and all of our accounts were constantly mishandled by management. I had only one review at that point, a year and a half earlier which was not reflective of my work, I was promised a follow up 3 mo review that never happened, and had not received a raise even though my work load was greatly increased, with duties that did not fit my role due to another person quitting unexpectedly who was never replaced. I regret this now, but my boss convinced me to stay by matching the offer that I received from the other company (although not matching what I requested) throwing in a few more vacation days (not as many as I requested) and giving me a dual role where I would now be an executive assistant and be able to create the change that I wanted in the office. Part of the reason I stayed is because I am pursuing my masters in another field and needed vacation time for field observation requirements. If I went to the new job, I would have to accrue my time and that would set me behind in my courses. I was also advised by the headhunter who referred me there to not let anyone know of my school plans because it would reflect negatively on me that I was not a dedicated employee. It was a difficult choice, but school being more important, I felt that staying at my current job would work out better in the long-run. I signed an agreement to my new position which guaranteed me a review on May 1, 2020 and a guaranteed minimum salary increase of a specific dollar amount on June 1, 2020. Here is where my problem lies. It is now nearly September and my boss has not given me my review or my raise. I first questioned him about it on June 30 and he told me to draft a new review template (even though we already have one) and he would review me. When I told him I was having trouble finding resources that would fit our office through Google, as he suggested, he seemed annoyed and said he would get to it next week. He never did and a month later he emailed me with links that my manager found for unrelated career performance reviews and told me to come up with something from that and he would review me. It is important to note that in my signed agreement there is no mention of my review being dependent on my drafting a new review template. It came up in conversation sometime after I was in my new position that my boss wanted new review templates, mainly to evaluate someone who was consistently late, absent, and not completing work, but it was never discussed as being necessary for my review. I questioned him on this and was told I have to make the review template if I want a review, so I complied. I have a friend in the industry who helped me out by providing me with her review template that was issued by her company's HR. I believe this should be a duty of HR for my company as well but we are a very small office and my boss has an HR company that he consults with but the employees cannot access that company. My guess is that to ask them to create a review would cost him additional fees so he is trying to get it from me. I drafted a great review template using my friend's review as framework and I completed my self assessment as was also required and sent it to my boss. He replied questioning why I didn't include everything from the links he sent me (which were reviews for college employees) he even made a spreadsheet comparing what I included on my review against what was in the samples. He suggested I look all this over and let him know if I was confident in my template, I stated I was. He then replied and told me that I must revise what I drafted and stated that I was being resistant against his "suggestions". I am looking for advice in how to proceed. Would this just be a matter for a labor attorney at this point? When I am assertive with my boss in asking for what is long overdue to me he has called me resistant and also "hostile". He has been taking advantage of me for a long time and I want to put my foot down but also want to choose my words wisely. I am being made to jump through hoops for a reward that I was promised. I am trying to hang on at this job while I complete my degree so leaving to start another job that I will not be at for very long is still not what I intend to do. Any suggestions or advice would be greatly appreciated!