Answer: he found out that marrying Ruth was part of the deal
In terms of the law, the land was tied to other obligations: if he wanted to acquire the land (always a desirable thing, and the reason for his initial agreement), he also had to provide sons in Elimelech's name, i.e. for Elimelech's posterity and inheritance, by marrying Ruth (since Naomi was too old) and having children with her on Elimelech's behalf. He could not have one without the other.
Since he could not 'redeem' the land without incurring the liability of another man's children, he opted out.
In the event of a son being born to Ruth, the child would legally be Mahlon's son, and Elimelech's grandson, and would inherit the land, in spite of the kinsman having 'bought' it from Naomi.
From Quiz: BBB Bible Series: Ruth
Answer: kinsman
In chapter 2, verse 1, Boaz is identified as a relative of the family and is described as a "man of wealth." His name means "in him is strength."
From Quiz: Ruth's Story
Answer: six measures
Ruth 3:15 "Also he said, Bring the vail that thou hast upon thee, and hold it. And when she held it, he measured six measures of barley, and laid it on her: and she went into the city." I am not sure what he measured the barley in, it only said "six measures" in the KJV of the Bible.
From Quiz: The Book of Ruth
Answer: All of these
In order to "redeem" a widow, slave, or orphan, the kinsman-redeemer had to be a relative who was both willing and able to pay the price of redemption. In the case of a widow, he had to marry her and any offspring would be given the deceased husband's name.
From Quiz: Ruth: A Touching Love Story
Answer: What is a shoe?
Ruth 4:7. In order to confirm the change in the redemption to another, a shoe was exchanged, signifying that the nearest kinsman forfeited his right as redeemer to the kinsman who was next in line.
From Quiz: Ruth Jeopardy
Answer: A relative
2:20
From Quiz: The Biblical Story of Ruth
Answer: the kinsman took off his sandal and gave it to Boaz
The removal of a sandal as a sign of 'sealing a deal' appears nowhere else in the Bible, yet it is recorded here explicitly as being the custom of the time.
It is probable that it was done on several other occasions, yet without comment, simply because it was accepted practice and 'went without saying'.
Note: The removal of a defaulting kinsman's sandal by an offended widow, as described in Deuteronomy 25:5-10, is a different custom - one of 'naming and shaming' rather than one confirming a transaction.
From Quiz: BBB Bible Series: Ruth
Answer: ephah
An ephah, found in 2:17, weighs about thirty to forty pounds, and some sources measure it around 1.1 bushel; others at .65 bushel. A mattock, mentioned in Isaiah 7:25, was a tool for digging and hoeing. Niter (nitre), as used in Jeremiah 2:22, is carbonate of soda.
From Quiz: Ruth's Story
Answer: Mahlon
Ruth 4:10 "Moreover Ruth the Moabitess, the wife of Mahlon, have I purchased to be my wife, to raise up the name of the dead upon his inheritance, that the name of the dead be not cut off from among his brethren, and from the gate of his place: ye are witnesses this day." Mahlon's name means "sick". It is interesting that Mahlon has this name because he died, leaving Ruth a widow before she married Boaz. Perhaps he died from a sickness as his name would suggest.
From Quiz: The Book of Ruth
Answer: Mara
"Call me not Naomi," she said, "call me Mara: for the Almighty hath dealt very bitterly with me." Naomi means pleasant, a meaning she no longer found appropriate, once she had returned home "again empty" (Ruth 1:20-21). This looks like it was a wry observation rather than a serious renaming: in the remainder of the book, Naomi is still called Naomi.
From Quiz: Ruth: A Touching Love Story