> if the mall has some unleased space, and is having problem filling it, you can cut a deal.

> and negotiated a 5 year lease for a percent of his gross sales.

> the landlord took the risk, could have been $0.

Gross sales. That's sales before expenses. Your brother-in-law took on literally all the risk in this scenario. Unless the zombie apocalypse started in that very mall, any store was going to sell some burgers and your brother-in-law was going to be paying out to the landlord before he had covered his own costs.