Albert Barnes Commentary Ruth 4:5

"Then said Boaz, What day thou buyest the field of the hand of Naomi, thou must buy it also of Ruth the Moabitess, the wife of the dead, to raise up the name of the dead upon his inheritance." — Ruth 4:5 (ASV)

Observe the action of the Levirate law. If Naomi had been the only interested party, she would have sold the land unencumbered by any condition, as the Levirate law would not have applied in her case.

However, there was a young widow to whom the possession of the land would devolve at Naomi’s death. This widow already had a right of partnership in it, and the Levirate law did apply in her case. Therefore, it was the duty of the גאל (gā'al) to marry her and raise up seed to his brother, that is, his kinsman. And he could not exercise his right of redeeming the land unless he was willing at the same time to fulfill his obligations to the deceased by marrying the widow. This he was unwilling to do.