The way the contract works is the club has to pay the agreed salary or make up the difference if the employee takes another job.
So if Rutten was due to earn $600,000 as a base and stay unemployed they pay the full whack. If he takes a job working full time at Woolworths on a checkout and earns $65,000 they owe him $535,000.
The obvious thing to do would be to sign him as a coach for $1 and let Essendon fund your coach but this has been raised before and the AFL has made it clear they will not allow a contract that is not in keeping with industry practice. So they will look at what other assistant coaches with similar skills and experience are getting and Rutten's deal would need to be in the same ballpark.
Effectively we might get him for a bit skinnier rate than normal but it won't be a huge amount. The only real winner financially is Rutten who will paid 600k to do a 350k job.