If you paid a scammer

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Did you pay with a credit card?

Contact the company or financial institution that issued the credit card. Tell them it was a fraudulent charge. Ask them to reverse the transaction and give you your money back.
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Did you pay with a debit card?

Contact the company or financial institution that issued the debit card. Tell them it was a fraudulent charge. Ask them to reverse the transaction and give you your money back.
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Did you pay with a gift card?

Contact the company that issued the gift card. Tell them it was used in a scam and ask if they can refund your money. Keep the gift card itself, and the gift card receipt.
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Did you send a wire transfer through a company like Western Union or MoneyGram?

Contact the wire transfer company or your financial institution. Tell them it was a fraudulent transfer. Ask them to reverse the wire transfer and give you your money back.
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Did you send money through a money transfer app?

Report the fraudulent transaction to the company behind the money transfer app and ask if they can reverse the payment. If you linked the app to a credit card or debit card, report the fraud to your credit card company or financial institution. Ask if they can reverse the charge.
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Did you pay with cryptocurrency?

Contact the company you used to send the money to and tell them it was a fraudulent transaction. Ask to have the transaction reversed, if possible.
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Did you send cash?

If you sent it by U.S. mail, contact the U.S. Postal Inspection Service at 877-876-2455 and ask them to intercept the package. To learn more about this process, visit USPS Package Intercept: The Basics

If you used another delivery service, contact them as soon as possible.