Skrill to Skrill send money transfers are instant and once received, they are irreversible.

Sometimes, however, you could still cancel such a transaction if it's still pending, in case:

• the e-mail or phone number of the recipient is invalid;
• the recipient doesn’t have a Skrill account yet. In this case, they will receive an e-mail invitation to register within 14 days and get the money;
• the recipient’s account is restricted, e.g. due to exceeded transaction limits, etc.

To cancel the transfer, simply select the pending transaction and click Cancel. This option is only available on web for now:

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Once the transaction is canceled, the funds will return to your available balance right away.  

I was scammed. How can I cancel my transaction?

If a transaction has already been processed, it cannot be canceled or returned/reversed. Contact the recipient to negotiate the case. As Send Money transactions are final and irreversible, make sure that you send funds to people you know and trust!