Spliting a lot with a mortgage

[Brad Gates]

I live in Boise Id and have a property that has a house next door with a large lot. I would like to buy the back half and access it from side. The city is ok with it, my surveyor sees no issues.  The owner has a mortgage on the property and will need the mortgage holder to release the property and let the owner sell it. What are the pitfalls we might encounter?What is the best way to get the bank to release their interest.  Thanks for your time.