Aborting/cancelling a transfer transaction I'm running Bitcoin Core and I'm 7 days behind syncing currently.

Last night, I initiated a transfer of some bitcoins to another person (He bought it from me) However, 4 hours later, he closed his account, and hence, the money he paid me was returned to him automatically without my approval. Since that is the case, I needed to cancel/abort the transfer.

As the bitcoin network is very busy at this time (there were over 10000 unconfirmed transactions), I have no idea whether my transaction will eventually get confirmed or not. 

I shutdown the Bitcoin Core app then deleted the mempool.dat file and restarted the Bitcoin Core app with the -zapwallettxes command so as to tell the Bitcoin Core app to delete the transaction. The app is currently rescanning.

Before the shutdown,  I can see that the transaction is still unconfirmed. 

UPDATE: Bitcoin Core app has finished rescanning, and the unconfirmed transfer transaction has came back.


How can I abort this transaction?
You can't cancel a transaction even if your node has not yet synced. Your transaction is immediately broadcast to the network.

If the tx is still in the mempool, you send the coins to yourself with a higher fee... and pray.