Primeiro de tudo, ele não cruzou com eles, estritamente falando.
O acordo era para ele ajudar o trio a invadir o cofre de Lestrange - NÃO para ajudá-los a voltar.
Griphook os levou até o cofre para que ele cumprisse 100% a sua barganha.
Aliás, ele até fez MAIS do que a barganha e ajudou-os a pegar a taça no cofre:
It was Griphook who had seen it [sword with the cup on it] and Griphook who lunged, and in that instant Harry knew that the goblin had never expected them to keep their word.
Agora, você pode se perguntar por que ele não foi além e ajudou-os a sair, o que NÃO fazia parte do acordo. Para isso, veja a próxima seção.
Não há prova direta de cânone, mas pode ser facilmente inferida:
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Bill Weasley disse muito explicitamente a Harry sobre as ideias de propriedade de Goblin:
“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. Goblin notions of ownership, payment, and repayment are not the same as human ones.
...
... However, there is a belief among some goblins, and those at Gringotts are perhaps most prone to it, that wizards cannot be trusted in matters of gold and treasure, that they have no respect for goblin ownership. -
A Espada era praticamente um dos principais artefatos feitos por Goblin.
“Wizarding arrogance again! That sword was Ragnuk the First’s, taken from him by Godric Gryffindor! It is a lost treasure, a masterpiece of goblinwork! It belongs with the goblins. The sword is the price of my hire, take it or leave it!”
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Griphook provavelmente tinha um pressentimento de que Harry pretendia manter a espada, pelo menos temporariamente. E mesmo que não, ele teria considerado um movimento natural para um portador de varinha, até mesmo um pouco mais desenvolvido como Harry.
Harry / Ron praticamente se entregou ao negociar:
“I want the sword. The sword of Godric Gryffindor.”
Harry’s spirits plummeted.
“You can’t have that,” he said. “I’m sorry.”
“Then,” said the goblin softly, “we have a problem.”
“We can give you something else,” said Ron eagerly. “I’ll bet the Lestranges have got loads of stuff, you can take your pick once we get into the vault.”
He had said the wrong thing. Griphook flushed angrily.
“I am not a thief, boy! I am not trying to procure treasures to which I have no right!”
“The sword’s ours—”E mais tarde - depois de uma óbvia pausa no intervalo:
Hermione frowned at the floor while he was speaking; he felt irritated at her, afraid that she might give the game away.
However, Griphook had eyes for nobody but Harry.
“I have your word, Harry Potter, that you will give me the sword of Gryffindor if I help you?”
“Yes,” said Harry.
“Then shake,” said the goblin, holding out his hand.
Harry took it and shook. He wondered whether those black eyes
saw any misgivings in his own. Then Griphook relinquished him, clapped his hands together, and said, “So. We begin!”... e mais tarde
The door of the bedroom opened and Griphook entered. Harry reached instinctively for the hilt of the sword and drew it close to him, but regretted his action at once. He could tell that the goblin had noticed.
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Portanto, no que diz respeito ao Griphook, esse movimento preventivo era 100% lógico e sensato.