1) The property appears to have a BMR (below market rate) deed restriction.
2) The City may provide a type of second mortgage that will typically need to fit into eligible subordinate financing guidelines.
Long/short, talk to your lender to make sure this property can be conventionally financed. If you have any questions, let me know.
It sounds like you may be looking at a City second program. Typically it is not a 20% loan, but many cities now have programs for those with very low to low income to allow them to purchase a home.
Those that take advantage of those programs will find the second loan is deferred for a period of time or until the home is sold.
Also typical for those types of loans, the City then has first priority when selling the property and for a time (about 30 years or greater), the home remains only sale-able to the same type of buyer with the City controlling the applicants for future purchasers.
It varies from City to City how the program is run - here is a link to the City of Fremont's Affordable Housing program to give you some insight: