My company discovered an error in the payroll system after

My company discovered an error in the payroll system after 16 months, that apparently affects only me, and have issued me a bill for $12,361.00 and are asking for repayment. What happened is I went from full time salary to part time hourly and the payroll clerk entered my rate of pay as 27.5/hour, instead of 27.5 hours per week. Since this all occurred when I was dropping my hours, my pay was reduced and I never noticed the error either. So, on top of receiving a bill for 12,361, they are not correcting and re-issuing my T4 for 2015 and apparently have no intention of correcting my T4 for 2016, as they say this is "for my benefit". I feel that is for THEIR benefit as they would need to redo their corporate taxes to fix it properly and they are looking to avoid that, but recoup their costs. They have said that if I dont pay, why would I expect to remain with the company? (I've been with them 5 years). They have not shown me where this money went, no breakdowns, etc. Also we are in an economic slump, and I don't have the extra money to correct their mistake. It just doesn't seem like everything is on the up and up. Employment standards have indicated that it is not my responsibility but I may be sued. What are my options here?