Visitor I

form 6252

I sold a home on a real estate contract in 2013.  There was an existing mortgage on the property.  The buyer paid me off in 2017.  The balance due on the real estate contract was $104,000.  The mortgage balance was $88,000 and was paid off.  The escrow company paid me the balance of $16,000.  What amount do I put on line 21 on Form 6252?  A tax preparer told me to put $104,000, but that does not seem correct...