Interesting question. My understanding is that post-purchase rebates are not taxable to the individual, so whatever you do would not necessary cause an IRS issue for your personal income reporting purpose (with proper documentation of course).

I myself would be slightly concerned with potential issue with the employer about reimbursement if I was in your shoes - does your company have a general policy or have you discussed with your manager (or the finance department)? I guess if it was a miles card, since the rebate value (miles) is not directly measurable, most would say okay for employee to keep; not sure what your accounting department would say about 2% cash which could be easily assessed if IRS one day insists on to re-value the true cost of business expense etc.