Feels high risk (fraud/abuse) with low reward, but if you want to ensure only paying commissions on actual customers then create coupons with some tracking means (serial numbers, bar codes, QR codes) that are associated with each of your promoters (employees) and force the the customer to provide the coupon at redemption. Store those in the cash register (or track digitally) and pay employee commissions at the end of the week (but track daily for rapid intervention in any abuse cases). You can run contests to increase payouts or provide other incentives to employee(s) with the highest redemption rate.
I'm sure there are more and better ways of driving traffic, but this approach should suffice for the use case you gave.
By the way, the easiest tracking means would probably be something like these raffle tickets:
https://jet.com/product/detail/16b7e...8c9ff330c1aa06