Since CRA has little information available online, it's always a good idea to contact the agency directly with your specific circumstances. This link provides different ways to contact CRA https://turbotax.community.intuit.ca/questions/1189856
Based on the information you have provided, it feels like the commission rebate will not be taxable. If the agent reflected the rebate in their agreement, then it would be nothing more than a lower commission fee- which would be treated the same as any other commission fee when it comes to Capital gains. If this is an investment property, you should just include the net fees in any calculations.
Thanks!
