Cash back rewards on business expenses can be dicey if your business gets audited by the IRS (at least if you're an S Corporation, where your personal and corporate returns are inter-dependant). A fiesty auditor, if they spot it, will say that the cash back should be booked as a discount taken on purchases and if you can't show the money going back into the company, it's unreported income and you'll be looking at seven years worth of taxes, interest and penalties on said cash back.