1) The statute of limitations for this, a cause of action based on a contract or agreement, is 6 years, so they are well within time to take legal action, including sending you to collections. 2) They can either sue you themselves or give the matter to a collections company or agency, which can then sue you if you do not pay.3) Whether or not it would have been better to have informed you when you left about how much to repay, where to send, etc. is irrelevant at this point: it does not affect their legal rights. 4) We can't say whether or not they will sue, since every person and company is different: some will deem it worth their time and effort to pursue the money, some will not. All we can is that they have the right to go after the money.