It looks like Apple Pay Cash, the feature that, among other things, will enable person-to-person payments through Apple Pay over iMessage, seems to be headed for a late October release date. In a footnote on its Apple Watch Series 1 site, the company says that the ability to pay friends using Apple Pay is coming in late October.
Apple Pay Cash is a digital debit card that will be kept in Apple Pay alongside your other cards. It's where the money that you've been sent through Apple Pay's person-to-person payments sits until you decide to do something with it. While you'll be able to send the money to your bank account, you can also use it to pay for things just as you would with any other Apple Pay card.