Signing bonus

[mpmontgomery]

I signed a 1-year contract with a school district and received a $1000 signing bonus. At the end of the year, I decided to change school districts, giving my previous school district appropriate notice. I was informed 2 weeks later that my signing bonus was contingent on me staying in the district for 2 years. I told them that I never received or signed any document that stating that information. I was told it was on a recruitment flyer and that they would dock my pay for $500 since I am not staying in the district.
If I was never made aware of this stipulation, nor sign any document regarding this, is the district still able to legally take back half of the bonus?