Paying back retention bonus

On Dec 14, 2012, I signed a retention bonus agreement for $10000. The contract states that if I leave within 6 months, I will pay it back. The contract also states that the employer may deduct money out of my final pay check and and/or any money associated with PTO time. This letter was from the companies CIO and CFO.

In April of 2013, I put in my two weeks' notice
I received the bonus on Dec. 31, 2012 and they deposited $5400 into my bank account. (after 401K and taxes). The human resources director then tells me that I need to pay it back, however, he states that the contract has an error in it and my PTO time has no cash value so it doesn't count. He also stated that he doesn't know if they can deduct money out of my paycheck or not. He claims he wants me to pay the full balance 3 months after my departure (May 3, 2013).

My argument is that they should take money out of my paycheck and PTO time like the contract says but now they are changing the terms on me. Deducting money from my paycheck will ensure that taxes, social security, and Medicare will be attributed to paying the retention bonus back. Also, my PTO pay equivalent is $2250. Instead, they want me to write a check for the full $10,000. The HR director stated that he will send attorneys or collection agencies after me if I don't pay back the full amount in 3 months (by Aug 3). The contract, however, doesn't indicate any timelines at all.
What are my rights in this case?