Is it required to charge interest on a seller financed deal?

[Jeff Hursey]

I am hoping to buy a property from a family member via seller financing. The family member is not planning to charge interest which sweetens the deal. I was discussing this with someone who had done deals like this before and she said the seller is technically required to charge interest. I then ran it by an agent who said the seller does not have to as long as that is part of the agreed upon terms. Any input is greatly appreciated, thanks!