Ruth 4:5

Then said Boaz
In order to try the kinsman, whether he would abide by his resolution, he acquaints him with what he had as yet concealed:

what day thou buyest the field of Naomi, thou must buy it also of Ruth
the Moabitess, the wife of the dead;
the wife of Mahlon, who was dead, the eldest son of Naomi, and so his widow, Ruth the Moabitess, had the reversion of the estate; wherefore the purchase must be made of her as well as of Naomi, and the purchase could not be made of her without marrying her; which, though no law obliged to, yet it seems to be a condition of the purchase annexed to it by Naomi, that she would sell it to no man, unless he would consent to marry Ruth, for whose settlement she had a great concern, having been very dutiful and affectionate to her; which is clearly intimated in the next clause:

to raise up the name of the dead upon his inheritance;
and so Naomi had another end to answer thereby, not only to provide a good husband for her daughter-in-law, but to perpetuate the name of her son, agreeably to the design of the law in ( Deuteronomy 25:5 ) .