Laws Concerning Levirate Marriage
5 “If brothers dwell together, and one of them dies and has no son, the wife of the dead man shall not be married outside the family to a stranger. Her husband’s brother shall go in to her and take her as his wife and perform the duty of a husband’s brother to her. 6 And the first son whom she bears shall succeed to the name of his dead brother, that his name may not be blotted out of Israel. 7 And if the man does not wish to take his brother’s wife, then his brother’s wife shall go up to the gate to the elders and say, ‘My husband’s brother refuses to perpetuate his brother’s name in Israel; he will not perform the duty of a husband’s brother to me.’ 8 Then the elders of his city shall call him and speak to him, and if he persists, saying, ‘I do not wish to take her,’ 9 then his brother’s wife shall go up to him in the presence of the elders and pull his sandal off his foot and spit in his face. And she shall answer and say, ‘So shall it be done to the man who does not build up his brother’s house.’ 10 And the name of his house shall be called in Israel, ‘The house of him who had his sandal pulled off.’
4 And in Jerusalem lived certain of the sons of Judah and of the sons of Benjamin. Of the sons of Judah: Athaiah the son of Uzziah, son of Zechariah, son of Amariah, son of Shephatiah, son of Mahalalel, of the sons of Perez; 5 and Maaseiah the son of Baruch, son of Col-hozeh, son of Hazaiah, son of Adaiah, son of Joiarib, son of Zechariah, son of the Shilonite. 6 All the sons of Perez who lived in Jerusalem were 468 valiant men.
If this is so then Boaz is saying that Ruth also has a stake in this field, and since she is the widow of a childless kinsman this involves marrying her to raise up a child for the deceased as well as treating it as a property matter.