Jamieson, Fausset and Brown on Ruth 4:6

Ru 4:6-8. He Refuses the Redemption.

6. The kinsman said, I cannot redeem it ..., lest I mar mine own inheritance--This consequence would follow, either, first, from his having a son by Ruth, who, though heir to the property, would not bear his name; his name would be extinguished in that of her former husband; or, secondly, from its having to be subdivided among his other children, which he had probably by a previous marriage. This right, therefore, was renounced and assigned in favor of Boaz, in the way of whose marriage with Ruth the only existing obstacle was now removed.