I am a realtor considering to offer rebates to my clients from my commission. I was wondering if the rebate is handled after the closing (after I receive my commission), am I still obligated to pay income tax on my entire commission or simply on the spread (commission minus rebate to customer).

I don't know the real estate business, but there's no logical reason to have to pay tax on something you rebated - regardless of how you handle it, it still reduces your net revenues.