Others have already told you that talking to a lawyer is still a good idea. If I may offer a personal story that illustrates that that is _really_ a good idea:
While I did my Bachelor's in CS, I was employed by a university (not the one I attended, but one that the project I worked on moved to after the Prof in charge switched universities) as a "student worker" type deal. My job was essentially a Jr. SWE.
A friend of mine also worked on that project, but he was ahead a bit further in his studies, so he already had a BSc degree, while I hadn't. Universities being universities, this meant that his hourly pay was a tiny smidge more than mine (think 50 cents/hour or something like that). Neither of us was paid very well, we both came out to about 10-12 €/hour.
After 6 months my contract was up for renewal. Along with the renewal, they included a modest pay raise to my friend's level. I naively thought that that meant they appreciated my work or something like that. All went well until the _next_ renewal was up.
The HR person responsible for student workers noticed that my "raise" had been in error because they assumed I had gotten my degree as well. None of their paperwork that I signed originally mentioned that. As "proof" that I "should have known" that the raise was in error, they sent along a scanned copy of a copy of a copy of an internal "wage schedule" that I somehow should've been aware of.
Their solution was to hand me a "new" backdated contract with lower hourly wages and told me to sign that to "just quickly fix this error" and told me to just pay back what I had "erroneously" received (signed contract stating the contrary nonwithstanding).
I politely declined because that's not how I think employment works. As a response they said "ah well, don't worry, we'll just take it out of your next pay check", which they did (without me signing anything).
At that point I called my mom and told her the full story. She immediately went "Alright, how do you want to play this? Should we talk to them or do you want to pull out the big guns?". I was sufficiently pissed off that I told her I want the big guns, she told me the info for my families' "lawsuit insurance" (The German term is "Rechtsschutzversicherung", basically cheap-ish insurance to help you pay for a lawyer in cases like this) and called them after we talked.
I called up a lawyer in town that specialized in employment law, had an appointment with him to tell him the story, he went "I can see roughly 4 or 5 reasons that they can't take that money from you, let me write a letter to them and we'll see how it goes".
The end result was that the university in my next paycheck included the amount they had initially reduced my previous check by, my higher-wage contract was renewed, and we never spoke of any of that again. I didn't get an apology or anything from the HR admin who had clearly messed up my contract and was probably trying to cover her ass, but that's fine with me.
Point being: talk to a lawyer, even just to get some advice or to have them write out a nice letter as to why what they're doing is not OK.