Salary increase with strings attached

I recently asked for a raise at my employer, where I’ve been for nearly two years in an outpatient treatment center. I’m a primary therapist and recently became EMDR trained and felt it would be appropriate to ask for a raise since I started practicing with more clients using this modality, something my company has said we’ve had a “deficit of”. My boss had agreed to provide me with a raise which was contingent upon me presenting my completed training certificate. I did that, now he’s changed his tune stating that “there’s a strong chance we will need you to sign a contract” in order to earn my raise. I have told him prior that I don’t sign contracts and paid for the training out of my own pocket, so could someone please enlighten me as to why he’s attempting to have me sign a contract for a raise? Is it about retention? Is this illegal? I’m kind of struck sideways about this.