It has been previously reported that Chris Jericho’s contract with WWE expired shortly after WrestleMania this past April, but he ended up signing a one-month extension that lasted through until late May to allow him to drop the United States Title. While he did work a three-show tour for WWE over the summer, he’s technically been a free agent for the better part of the last six months, which is how he was able to sign to do the NJPW match with Kenny Omega.
While some may question what Jericho’s current relationship with WWE is right after they declined his request to have NXT talent on his cruise next year and of course, his association now with NJPW, the two sides are still said to be on good terms and the expectation is that Jericho will be back in WWE sometime in 2018.
It is also possible that Jericho extends his NJPW deal to do a second match, but as of this time, it is said that the deal is only for one match.

Source: Wrestling Observer Newsletter