Tuition Repayment Clawback

Hello,

I am leaving my employer who I have received tuition assistance from, which had been returned back to me as taxable income. In accordance with their policy, I will owe back the tuition they paid within a certain time span. I am being asked to return the net amounts to the company over the past two years. While I am not returning the gross amount, as I understand it I have still paid taxes on that amount. Am I correct that I would need an adjustment? Is there a way for me to adjust my prior years' tax returns? Do I need my employer to correct my W2s? They are being a little difficult about it. I am unsure what to do going forward.