Psalms 108:9 Moab is my washpot; over Edom will I cast out my shoe; over Philistia will I triumph.

The LORD said, "over Edom will I cast out my shoe".

Edom is Esau, the elder twin brother of Jacob, who despised his birthright:

Genesis 25:30 And Esau said to Jacob, Feed me, I pray thee, with that same red pottage; for I am faint: therefore was his name called Edom.

  31 And Jacob said, Sell me this day thy birthright.

  32 And Esau said, Behold, I am at the point to die: and what profit shall this birthright do to me?

  33 And Jacob said, Swear to me this day; and he sware unto him: and he sold his birthright unto Jacob.

  34 Then Jacob gave Esau bread and pottage of lentiles; and he did eat and drink, and rose up, and went his way: thus Esau despised his birthright.

So, could the phrase, "over Edom will I cast out my shoe", be related to that, along with?

Ruth 3:12 And now it is true that I am thy near kinsman: howbeit there is a kinsman nearer than I.

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.

  6 And the kinsman said, I cannot redeem it for myself, lest I mar mine own inheritance: redeem thou my right to thyself; for I cannot redeem it.

  7 Now this was the manner in former time in Israel concerning redeeming and concerning changing, for to confirm all things; a man plucked off his shoe, and gave it to his neighbour: and this was a testimony in Israel.

Esau/Edom, the elder kinsman, despised his birthright, similar to the nearer kinsman of Ruth rejecting his.

Except in that case, the kinsman drew off his shoe.

Whereas the LORD said that he would cast his shoe (shoe, appears to be use anthropomorphically, here) 

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