Scammer sends you a sum of money from a stolen/compromised account. They say 'Oops, sorry - can you send it back to me at newaccount because my other one is overdrawn/locked/lost'. You 'refund' them and then PayPal says "BTW, you had £150 of stolen cash in your account from stolenaccount, we want it back thanks.". Now the scammer has your £150 cash, and you're down a second £150 repaying PayPal/the original account holder for the money the scammer originally gave you. Because it's in two transactions, PP say 'You received stolen funds in transaction 1, you separately paid a totally different account £150 - nothing to do with us, you still owe stolenaccount £150.'...
Yeah I believe that is the scam
If it was me I'd just let it sit there.
If it's a genuine mistake (it happens!) then the person will surely contact you. Then it leave it up to paypal to handle.