I'm a bit out of date on HR but my suggestions :
Your risks to post a sample docuement with names deleted, for suggestions here are low --but not zero if others are reading your mail.
Present or absence of an end date is usually a critical factor in determining if an employment agreement is a contract or just an nice letter --in your post it leans towards a contract and the elements of a contract may well exist.
To the extent that you crafted the contract and things are ambiguous the prevailing view may be that's your problem.
If you breech the contract you may have a big problem.
But that's not the only way to look at it. In general you can work a salaried employee ungodly hours -literally 24/7 subject to a few minimum wage issues and issues as to if they are truly salaried exempt --now I'm not suggesting you go to an overt extreme that has obvious intent to force a breech --but if you instruct employee to show up a couple of hours earlier than previously and you hold him over a few extra hours and come the weekend you require some Sunday work and you assign some tasks that he doesn't like --and he refuses to show or leaves after being direct otherwise and/or refuses the assigned tasks have the table leaning your way --a no show or a walk out might well be a quit -breech on his part .
I've seen $100/hr equiv salaried staff with PhD's assigned de minimus duties normally covered by $8hr hourly staff--so be it. If a staff lawyer or accountant is directed to tidy up the office including washrooms and lock up --she/he better do as directed!
I've seen a salaried person given a directive to get these 15 tasks done , stay until you have finished same
I've seen staff person direct to report at 7AM at obscure outpost # 6 or to report for unspecified duties (janitorial) at 6PM.
How far you can push it is a bit of an art but you can make things very uncomfortable for staff w/o treading into hazardous workplace issues. If they blow up and quit--eureka I've seen people give the boss an obscene salute and walk out over the strangest of issues. eureka.
I've also seen some totally nonproductive staff just sit there collecting paychecks because the boss was afraid to act...so be it.