Harry Potter and the Deathly Hallows

Yes, I read it.
If you have not read it, go away now.

For the rest of you, Somebody, please try to explain what seems to me to be a glaring plot hole:

• At one point in the action, a weapon is lost at Gringott's. Traded, you might say, to the original owners, and so done by Harry, who was arguably the rightful owner.
• Much later, that self-same weapon is used in the final battle.
  

Where did it come from? Yes, it was pulled "out of the hat" as it were, but didn't the Goblins have it? Were they not the rightful owners, both under their own traditions and by virtue of the fact that Harry gave it to them? Was it not Harry's to give? Or was the need so great and the valour so strong that its enchantment overrode these other considerations?

Theories?