Who knows how to cancel payment order at paypal?

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Lately i purchased $28.50 at xoom to transfer it to near pick up money transfer pawnshops, now I suddenly notice that my transaction with them is cancelled.. Now i don't want to try it anymore so I decided to cancel my transaction in paypal to xoom because it is still pending, does anybody here experienced the same thing? Please teach me on how to get my money back, thank you mylotters..