In Harry Potter and the Half-Blood Prince, Chapter 17, Dumbledore and Harry are using the Pensieve to experience the vital Slughorn memory (which was tampered with), and Harry is instructed to collect the untampered memory.
My question: How did Dumbledore collect the tampered one in the first place?
In that same chapter:
“you don’t need me — you could use Legilimency . . . or Veritaserum. . . .”
“Professor Slughorn is an extremely able wizard who will be expecting both,” said Dumbledore. “He is much more accomplished at Occlumency than poor Morfin Gaunt, and I would be astonished if he has not carried an antidote to Veritaserum with him ever since I coerced him into giving me this travesty of a recollection.
Is this implying that Dumbledore used Veritaserum to force the memory out of him? I suspect not, given that it was tampered with (i.e. not truthful). So, what did happen? How did Dumbledore coerce him?