Your employer could potentially recover the pay differential for the past 2 years if they can demonstrate by a preponderance of the evidence (more likely than not) that their written policy *at the time* was such that you did not qualify for such payment under clear, objective criteria. If they can't demonstrate some objective policy or criteria that they can point to, however, they should not be able to recover the money--without a written policy with clear criteria, the pay to you was essentially discretionary on their part, and an employer is not allowed to later think better of and recover discretionary pay.