Apple has provided a more detailed roadmap for upcoming operating system updates.
That includes the news that its iMessage P-to-P feature will be coming out later this fall, according to

The new payment feature will not be deployed on Sept. 19 as part of a series of new features that Apple is expected to announce as part of a new operating system, iOS11. iOS11's more broad upgrades will include improved photos, augmented reality, an update for Siri and a "do not disturb" feature for driving.
iMessage payments will be Apple's competitor to Venmo and PayPal, and were first announced in
iMessage funds will come from cards already stored in Apple's digital wallet, and the service will work with Siri, allowing payments via the personal assistant. Venmo, Square and other P-to-P apps already work with iMessage, though Apple hopes its large number of existing card accounts will provide a readymade market that can easily add the new payment feature.