Why Did Dumbledore Let Harry Give the Remains of the Diary Horcrux to Dobby?

J.K. Rowling confirms that Dumbledore knew the destroyed diary was the remains of a Horcrux;

JKR: "In essence divided," exactly, the "essence" being the soul. So Dumbledore knew all along that he must have- well, he suspected until Chamber of Secrets, and then at the point where he saw what was clearly the remains of a Horcrux, in other words, the diary, he thinks, "Okay. There you go. And not only has he definitely done it, but he's got to have more than one, because he's treated this one very casually."

_{[POTTERCAST - THE LEAKY CAULDRON - 12.23.07]}

_{Yet Dumbledore allows Harry to take this very dark magical artifact and return it to Lucius Malfoy (in order to free Dobby):}

_{‘Professor Dumbledore,’ he said hurriedly, ‘can I give that diary back to Mr Malfoy, please?’}

‘Certainly, Harry,’ said Dumbledore calmly. ‘But hurry. The feast, remember.’

Harry grabbed the diary and dashed out of the office.

Chamber of Secrets - page 248 - Bloomsbury - chapter 18, Dobby's Reward

Why did Dumbledore allow Harry to take the Horcrux before he had fully examined it, especially as Dumbledore knew it was a Horcrux? The Peverell ring had a lethal curse placed on it for anyone who tried to put the ring on. Why didn't Dumbledore check the diary for additional curses or Dark Magic before giving it to Harry? I would have thought Dumbledore would have wanted to keep whatever Horcrux relics he found.

Also, do we know what happened to the diary after Harry gave it back to Lucius Malfoy and Lucius threw it aside? Are there any instances in canon that place the diary back in Dumbledore's hands?

Answer

Dumbledore is pretty much one of the smartest, most cunning wizards around.

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  Even without the Legilimency, he probably figured out WHY Harry wanted to do this. Remember, he's one of the rare Wizards who DOES see and "know of" house-elves. He explicitly criticizes Voldemort for that, and he was the one telling Sirius to stop mistreating Kreacher.

  
  


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  And even if he didn't, he was willing to trust Harry's brain, instincts and courage in pretty important things (like thwarting Voldemort at the end of Year 1). It was 100% obvious that Harry had a plan, so why stop him when he's likely to succeed?