So the guardian said to Boaz, “You may acquire it,” and he removed his sandal. ¹ 

(Now this used to be the customary way to finalize a transaction involving redemption in Israel: ¹  A man would remove his sandal and give it to the other party. ²  This was a legally binding act ³  in Israel.)

then his sister-in-law must approach him in view of the elders, remove his sandal from his foot, and spit in his face. ¹  She will then respond, “Thus may it be done to any man who does not maintain his brother’s family line!” ²