Friend forced to sign a contract saying they’d stay with employer for a length of time, is this legal?

Hello! My friend, who lives and works in Minnesota, was forced into signing a document saying they would continue to work for a company until a certain date (example - August 202x) in order to be paid for extra work they’d done after 90% of the department was fired/quit. They are now wanting to leave the company but the document/contract they’d been forced into signing is not up for 6+ more months and they haven’t received ~50% od what they were owed for the extra work they’d done as promised.

I’m trying to help them navigate this so they can leave a toxic work environment, one I also worked within before leaving last spring. Minnesota is an at-will employment state, which (I believe) means you can quit without giving reasoning and be fired without reasoning. I haven’t seen the contract they were forced into signing myself, but from my understanding based on what they’ve told me it states that the “signee would stay with the company until date” and I only recently learned of the monetary incentive.

Thanks in advance!

Location: Minnesota