TBH it's pretty Morton to have an extension clause worth an unspecified amount. Another way of phrasing that would be 'not having an extension clause.'
Who ****ing knows what's happening.
It's possible that it does specify an amount, but it's optional for the player for accept the extension or not.
So if the player meets the criteria for the clause to trigger then he's at least guaranteed the security of a contract for next year, but still has scope to seek a better offer from Morton or elsewhere.