If the employer knew (or should have known) that your wife was relying on this representation of reimbursement in making her decision to accept the job in the nearby city and, knowing that, nonetheless represented that she would be reimbursed for mileage, then the representation may be legally enforceable under the theory of "promissory estoppel." However, to enforce it, your wife would have to sue her employer, which has its own costs, monetary and otherwise. You and she need to decide whether suing her employer makes sense for her.