If the buyer has filed the dispute through his credit card company it will be a chargeback through PayPal and there's no "up front" question of the details, PayPal has already refunded the money back to the buyer's credit card, and has taken the $$$ out of the sellers account or locked his account if there weren't enough funds to cover the dispute.
PayPal can't exist without MC/Visa/AmEx, thus PayPal will NEVER take a position against a credit card company. How PayPal even learns that there has been a dispute filed is that the money has been taken back from PayPal by the credit card company. That is the extent of "negotiations" or "notice". PayPal now goes back against the seller to restore their funds (which again, are 100% gone as soon as the dispute hits PayPal from the credit card co) and all this happens before there is any "talk".
These threads are often about "what should happen" rather than "what will happen". I've found that in these matters it is best to deal with what is rather than what should be.
As a seller in a PayPal transaction one must always ship to a confirmed PayPal address (in the US) and have positive proof of delivery or you are a loser in any dispute. If you have done what I suggest, then you are eligible for PayPal's chargeback insurance. If you are eligible for the insurance, PayPal knows you have satisfied these requirements and they could actually be out the funds, they will treat the seller's side of the transaction seriously. Otherwise, the buyer got his $$ back from his credit card co, the credit card co got the $$ back from PayPal, PayPal got the $$ back from the seller and the seller is out. Done deal.
Kevin