Subject To financing question

[Rosston Smith]

I'm looking for clarification on Subject to. When subject to financing is used, is the equity the seller has put into the property forfeit? Meaning, is the seller paid for the equity they have put in the property so far?

For example"

The mortgage on the home is $100,000.
The seller has paid $20,000 of it down.
The remaining mortgage value is $80,000.
I assume the $80,000 mortgage.

Does the seller lose the $20,000? What would entice them to give that up?