I to agree with @WayneSchulz is directing the client to their CPA for taxability guidance. Regarding how to record, I would record the full amount incurred from a vendor as an expense and then record as offset to a credit card incentive discount earned account as a type of revenue. The credit does not relieve the purchaser from sales tax liabilities on the purchase (or use tax if applicable). The discount is taxable for income taxes to the extent that it reduces the amount of the net expense incurred.
Since we most likely all participate in some type of rebate or loyalty program with one or more credit card companies, we all have to deal with this to one extent or another. If your a small business owner like me it doesn't matter whether you use them for your personal benefit (vacation travel to Hawaii is a common use in the Slavin household) or to reduce your business expenses, the net result is the same; more money into your pocket at the end of the year.