Owner Financing Clarification

[Neiman Lee]

How would this situation work? 

Was wanting a little clarity for owner financing. I understand that the owner of a property that I want to buy can actually fund the purchase where I would make monthly payments to the seller rather than a bank if they own the home free and clear( no existing mortgage). Lets say the owner of the property still has an open mortgage on a property and I would like to buy and finance the deal through the seller. The seller has to pay the loan back to the lender to avoid foreclosure. How would the seller pay the loan back and avoid foreclosure? Lets say the seller has paid $50K of a $100K mortgage. Does the seller have to come up with another $50K?