Not unless there was some agreement in place (e.g. some document or notice or written agreement provided to you which you signed and therefore accepted the terms of) stating that they could get interest in addition to repayment of the principal sum or amount of the error. Otherewise, they are limited to recovering from you any amounts you actually cost them (e.g. say you lost $500 somehow of the employer's money: you have to repay them that $500).