The agreement doesn't say so (iOS Schedule 2 v21):
"You may change the price tier for any Licensed Application at any time, at Your discretion, in accordance with the pricing schedule set forth on that Exhibit C as updated from time to time, using tools provided on the iTunes Connect site. As Your agent and/or commissionaire, Apple shall be solely responsible for the collection of all prices payable by end-users for Licensed Applications acquired by those end-users under this Schedule 2."
Full Story: https://medium.com/@marcmasat/apple-itunes-support-aa2fb0f9c4c