Can I just sell it off to a collection agency.

Quote from @James Wise: The term "cross selling" is used when you sell related products.

One day he calls me and says he and his wife want to sell a building they own and they thought of me. 

Most lenders sell their loans out.Nobody wants a loan where the asset is underwater every month.The real question is, why is a property worth over $1M only producing $4,000/mo.You should consider raising rents as this seems significantly below market.

Unless you want to start selling real estate for a career, there is no need for it.

If you can have better cashflow then sell.

My advice would be fix the dog, and sell the puppies or adopt them out to a good home. 

If it’s in California they will chase you for state income tax if you ever sell your new property, even if you exchange out of state. 

Condos are their own niche so make sure your agent is experienced with buying/selling condos in your market3.

Maybe in a year you could sell the note.