If it's potted you can plant it anytime you'd like. If it's really hot where you are I'd wait until it starts cooling down some or find a way to shade it until it's had a chance to settle in. You'll want to plant a tree peony deeper than a herbaceous since most are grafted, probably around 4-6 inches deep.

If you decide to wait until fall to plant just keep it watered routinely and give it more shade than you're used to for peonies, the pots can dry out really quickly and get very hot. Congratulations on your find!