If you are not paying income tax, under the fair tax system, why would you get a rebate on expenses? Getting a deduction on expenses is to offset the income tax. Without it, why do you need a deduction?
I don't understand your question.
The rebate is not on the income tax, it's on the sales tax. See the faq here: http://www.fairtaxvolunteer.org/smart/faq-main.html#3