It has always been the case that if a dispute is opened the funds are frozen in Paypal. Once the dispute is over the funds will either be released or they will be refunded to the buyer if you lose. If it is a change of mind return and your return policy says return of item price only then the whole payment will be frozen and once tracking shows you have it back the buyer will be refunded the correct amount and the rest will go back to being available in Paypal.

If the buyer does not escalate the claim within 20 days then the dispute is closed in your favour.