Hi BP Family, what happens to the amount a Seller owes on his mortgage?

a) Buyer must pay off the loan before acquiring the property? 

b) Bank allows Buyer to take over the mortgage?

For example:

After repairs Value = $150,000

Total Seller Mortgage Amount = $100,000

Seller Equity = $60,000

Amount Seller Owes the Bank = $40,000 <--- What happens to this amount?  

Wholesaler Purchase Price = $70,000

Rehabber Purchase Price = $80,000

Repairs Cost = $20,000