"Goblin notions of ownership, payment and repayment are not the same as human ones."

From pages 417-418 of the paperback edition of Harry Potter and the Deathly Hallows:

"Then I have to say this," Bill went on. "If you have struck any kind of bargain with Griphook, and most particularly if that bargain involves treasure, you must be exceptionally careful. Goblins notions of ownership, payment and repayment are not the same as human ones."

Harry felt a slight squirm of discomfort, as though a small snake had stirred inside him. [Harry has just promised to give Griphook the Sword of Gryffindor, but deliberately didn't tell him when. Griphook settles the matter later on by simply taking the sword and disapparating with it].

"What do you mean?" he asked.

"We are talking about a different of being," said Bill. "Dealings between wizards and goblins have been fraught for centuries - but you'll know all that from History of Magic. There has been fault on both sides, I would never claim that wizards have been innocent. However, there is a belief among some goblins, and those at Gringotts [Bank] are perhaps the most prone to it, that wizards cannot be trusted in matters of gold and treasure, that they have no respect for goblin ownership."

"I respect -" Harry began, but Bill shook his head.

"You don't understand, Harry, nobody could understand unless they have lived with goblins. To a goblin, the rightful and true master of any object is the maker, not the purchaser. All goblin-made objects are, in goblin eyes, rightfully theirs."

"But if it was bought- '"

"- then they would consider it rented by the one who had paid the money. They have, however, great difficulty with the idea of goblin-made objects passing from wizard to wizard. you saw Griphook's face when the tiara passed before his eyes. He disapproves. I believe he thinks, as do the fiercest of his kind, that it ought to have been returned to the goblins once the original purchaser died. They consider our habit of keeping goblin-made objects, passing them from wizard to wizard without further payment, little more than theft."