I'm not close enough to how these negotiations work, but we won't have been the only club trying to sign him.
If a player/his agent wants a term like that, I imagine it's one most clubs would accept. If you're banking £100m+ there's pretty limited downside.
I'll stress again though: it's just...
My interpretation of the wording was that his contract had a promotion clause in it, triggering a year extension:
"In addition, the club has extended midfielder Richie Towell's deal to 2019 - following promotion to the Premier League."
Maybe I'm wrong though.