Canada only.

If a Realtor gave a commission rebate back to the buyers of a property does the Realtor have the claim the entire commission as income, or just the portion he/she retains?

Who is paying the actual rebate you or the broker? If the broker is paying the rebate then it should be the net difference minus the rebate that is reported on your T4. If this is the case than I expect the gross commission would be paid directly to the broker and you would be paid the difference between the gross amount and what the broker takes out.
If you are directly in the Business of selling houses and are not employed by anybody than you would simply complete a Business return for taxes and declare all your gross revenues and subtract all your expenses including rebates.