What is the HUD $100 down payment program?
There are 2 different $100 down payment programs that buyers might be able to take utilize:
First is the Good Neighbor Next Door program, searchable on HUDHomestore.com but limited in applicability. See more information on this option here.
Most likely what you are seeing reference to is a FHA financing program for HUD Homes for FHA buyers. You can learn more here.