1. Downloading electrum right away (if you do not already have it on hand, which is handy in general and for these kinds of situations).
2. Exporting the private key from bitcoin core and importing it to electrum (from memory you can go to the developer console and use dumpprivkey).
I see. Well, that is unfortunate.
I suppose the lesson here maudo is that if you are dealing with someone that you have not dealt with before in a transaction without some form of escrow system, then the best thing to do is to use an updated client and ensure RBF is enabled. This is still not a sure-fire way to prevent being scammed though, as I am sure they will just hold you online until funds are confirmed...so overall, just take better precautions. Sorry for your loss.