I signed on with a large Canadian tech company about 6 years ago, and submitted my resignation (company pays well, but new management was installed about a year ago, and they're very abusive, vulgar, and ineffective). About 3 years ago (under my old, competent/kind boss), I received a pay raise from $150,000/yr to $175,000/yr (i.e. on my "awards statement").
However, when I logged in to the HR web portal a few weeks after being awarded the raise, I noticed that I was instead being awarded $195,000/yr. I honestly didn't think about it, since it was a really good year for me (i.e. I saved a large project, and literally millions for the company), so I thought this was deliberate, and continued working diligently at my job (I'd still be there if management wasn't so horrible).
My employer has informed me that this was a mistake (they won't communicate it in writing), and are holding my final pay cheque and owed/banked vacation time, and sent me an invoice demanding the "excess" (roughly 3 years @ $20,000; amounting to a little over $60,000, plus a 5% annual interest rate compounded annually, pushing it up to about $66,000) before they will "honor my letter of resignation". They keep e-mailing me these forms they want me to sign that will provide a 6 month "payment plan", and couriered copies of it to my house.
Is there anything from employment laws/standards I can cite to get them to go away, or can they seriously demand I pay them this money?