Vacation time agreed to at time of hire then changed after quitting?

I was rehired by my old employer after being gone for about 1.5 years (I was there for 7 years prior). My rehire offer letter stated that I was to return with 4 weeks vacation. I only worked 5.5 months and decided to leave to return to my previous employer. I gave a 2 weeks notice with a resignation letter and in my exit interview they were not happy. They said they could take me to court because I told them I would stay for at least a year and agreed to sign something stating that fact (which never happened - I signed nothing). They did state that it would not be worth the time or money to go that route, but now they are coming after me for compensation for vacation time that I used and supposedly did not earn. According to their vacation policy, I only accrued 2 days and used 7 and now they want the money for the 5 days I did not earn. My question is, if I was hired in at 4 weeks vacation to use in 1 year, how can they say I only get 2 days after almost 6 months like I was a new hire and none of this was negotiated beforehand?