section 8 rent question

I plan to rent my investment house to a section 8 family.  If my rent is $1500 and section 8 will pay for $1000, is it legal to receive the additional $500 from the tenant separately?  I know lots of people doing this, but I also heard that it's ilegal.  If it's ilegal, then if the tenant doesn't pay the money, how do you protect yourself?  Can somebody answser my question?

Thank you.