Your friend should be doing a "partial refund" inside of the PayPal system. That is a special feature in the seller controls. If he can't figure it out, he can do a full refund and then you can make the correct payment this time.
The above advice assumes he is either a friend or a businessman who is happy to work with you (the vast majority are, business isn't about chiseling every possible dime i.e. "never give the money back": it's about cultivating relationships and building social reputation).
Now if you are fighting each other, then you need to go to either the PayPal refund system (which won't work if money was sent as a "gift") or, you need to go to your credit card company and do a chargeback. American Express is aggressively consumer-friendly and will almost certainly side with you. Visa and Mastercard are not quite as much, again you will likely win but it's not as sure.
A chargeback doesn't wipe out the debt, it just cancels the payment method. You still owe whatever you owe.