Do I have to pay back all relocation monies if my employer did not fullfil its part of employment contract?

My employer paid me 15K(paid at net) plus moving expenses (5200) to relocate. I was obligated for 2 years. Payback was to be 50% of gross if I left between 12 and 24 months, anything less than 12 months 100%. I negotiated a review in 6 months and another at 12 months in my contract. I had worked for them 15 months with no review. I received an offer from a previous employer that I took based on conversations with my HR manager. He said not to worry about the relo money, that they had never gone after anyone for the money, just to make monthly payments $50 or $100. While I do feel obligated to pay some amount back, I think since they did not fulfill their obligations to me, which cost me two raises, I do not owe the entire 50%. Please help me understand my rights and what I owe them.