What law deals with relations between individuals.

The owner of a one-acre parcel of land with a small house on it rented the property to a professor of a nearby college at a monthly rental of $500. several years later, after the professor got tenure, the parties orally agreed that the professor would purchase the property from the owner for the sum of $60,000, payable at the rate of $500 a month for 10 years. they agreed that the owner would give the professor a deed to the property after five years had passed and $30,000 had been paid toward the purchase price, and that the professor would execute a note secured by a mortgage for the balance. the professor continued in possession of the property and made all monthly payments in a timely fashion. when he had paid $30,000, he tendered a proper note and mortgage to the property owner and demanded that she deliver the deed as agreed. the owner refused because valuable minerals had been discovered on adjacent parcels in recent months, causing the value of this parcel of land to increase to 10 times its former value. the professor brought suit against the property owner for specific performance. if the court rules in favor of the property owner, what is the likely reason?