PayPal is notoriously a pain in the ass. I've had them "permanently limit" my account with $5500 stuck in there in the past. They said it was locked in case anybody wanted to file any disputes. Then after 6-12 months (I can't remember) they released the funds to me. Had similar things happen before and have lost plenty of disputes due to one reason or another.
They really do side with the person sending the money more often than not. For that reason, PayPal is one of my least favorite means of financial transactions.
Hindsight is always 20/20 and I would have been very suspicious of somebody sending me that much and requesting I send them money somewhere else... but it is what it is. I would go after whoever initially sent the money and do whatever you can to get that money back from them. It's possible they submitted a chargeback or they could have even used a compromised paypal account. It would be difficult or impossible to tell for sure.
I'm not sure how far PayPal will go in pursuing you to get that money back, but I feel like you will ultimately be held responsible for this negative balance.