Typically sales people will get a post-departure commission check for any deals that close within 30 days after their last day. Sometimes there are special considerations, such as a deal that is in-process of closing, but you both know is going to take an abnormally long time to close.
Of course, the flip side of this is that the sales person is likely going to "ride the pipeline" for their first few months, closing deals that they had minimal influence on, so in some ways they are getting paid up front in a manner. I have been known to ask sales people that get too hung up on weird commission nuances around departures if they are willing to forego commissions on any existing pipeline deals ;) (note: savvy sales people can answer this question in a way that proves their value add. Less savvy sales people will be a bit deer in the headlights, or indignant, when you suggest they don't get paid for deals they didn't originate and control).
If not, he said, what happens if he is getting tons in commision and we decide it is cheaper for us to fire him then to continue paying commissions.
I understand his concerns, but this is a slightly odd question. It implies he is going to be selling RMR-related services. Typically for deals that have ongoing revenue, some account TLV is calculated by the business, and a 1-time commission is paid based on that, often after a minimal time period to make sure the account "sticks" (typically 2-6 billing cycles, but it can vary a lot). Therefore it is never cheaper for you to fire him, as he is only getting paid for new accounts, which are mutually beneficial to you both.
If he is expecting monthly commissions for all active RMR accounts, the amount per month is going to be pretty small, probably $1.00/account or less. It would take a while before you really had to test the results of such a scenario.
Additionally, most businesses do not set out to limit their growth. If someone can deliver such results that you would consider firing him for the cost-savings, wouldn't it make more sense to KEEP a person like that on the expectation that they continue to deliver new accounts? Maybe even promoting them to run a region or division to help further accelerate growth.