I would say it depends on how much "selling" the salesman is actually doing. If you are paying them to sell the account with little effort from yourself you'll should pay them a good portion of the initial service, or work out a per cleaning deal over several cleanings. The latter may work best for your preference, since the longer the account is serviced the more money they get for the sale. Sounds like a better title may be Account Manager for the duties you described.
I heard Chris Gibson only accepts payment in gold ingots.
If they are just generating leads for you to follow up with, I'd say a burger and fries should be sufficient. You might offer a "super size bonus" for every 10th lead.
Pay him based upon what he sells. No big fat salerys!!
Give him a percentage of the first service and then a residual each time you go back to that account (as long as they continue to be a customer).
The percentage amounts you pay would depend on what you want to pay him, your prices, how many accounts you have, how busy you are etc.
Be careful - if you have a salesman with no understanding or background in grease you may wind up with a bunch of accounts you do not want/ bid too low.
Be ready to pay him A LOT. Many people (including the owner of my company) ask me why I pay my saleseman so much money. My answer is simple: he makes me money. His paydays are a bit of a pain, especialy because he likes to get paid in cash: