Ruth 3:13
Tarry this night, and it shall be in the morning, that if he will perform unto thee the part of a kinsman, well; let him do the kinsman’s part: but if he will not do the part of a kinsman to thee, then will I do the part of a kinsman to thee, as the LORD liveth: lie down until the morning.
Boaz promised that the nearer kinsman would have the first opportunity to perform the kinsman's duty to Ruth, and that if that man declined, Boaz himself would perform it. The proper order was that the nearest eligible kinsman had the prior claim, with the right passing to another capable kinsman by blood if the closer relative would not act. Boaz acknowledged that another man was nearer kin than he was, yet stood ready to redeem Ruth if that man would not.
He solemnly confirmed this promise with an oath by the Lord, swearing by the Lord that he would do the kinsman's part if the nearer man declined. He told Ruth to lie down and remain with him until morning, when the matter of kinship redemption would be settled. The waiting until morning was tied to giving the other relative the first opportunity, with Boaz pledging before the Lord to act if that opportunity was refused.
Under the law of redemption in view, the right to redeem a person or an inheritance belonged to a near, able blood-relative. A nearer relative took precedence, so that the nearer man had to be given the chance first before another kinsman could step in. If the nearer man would not redeem, the duty fell to the next capable relative by blood who was related to the one in need of redemption.
The duty Boaz had in view was to redeem Naomi's inheritance and to marry Ruth, as the kinsman's duty required. In doing so he would be acting on behalf of both Ruth and her deceased husband, fulfilling the obligation of a nearest kinsman toward them. This arrangement kept a deceased man's inheritance within his family, since the one marrying the widow would have the firstborn counted as heir to the dead man, preserving the family homestead.
The kinsman's role serves as a type of Christ. The kinship principle teaches that humanity's Redeemer had to be related to those He would redeem, sharing their human nature and kinship. At the same time He had to be the Son of God.