Requesting help!

A knife from CS:GO ? I doubt so

Maybe Steam Support can revert trade...but I think they do in cases when games get revoked, here is your fault.

used the knife to rob someone (?

if you trade on the steam's trading window, that means, you trade your knife for other inventory elements, then yes. you can revert the trade... if you trade for paypal or keys, then no, you are fucked.

Thanks for your support guys. It happened in the trade window and if I don't get my friend the knife, I will have to pay him 60 dollars back in real life.

When it comes to PayPal and codes, alway, and I mean ALWAYS let them go first.

You're screwed basically. Paypal is incredibly risky, you should trust no-one. They can even chargeback scam you after going first. I only do Paypal with 120% trusted traders and on very rare occasions.

Looks like Steam won't be helpful.