I'd say this depends very much on the particulars of those "deferred wages". If de Jong agreed to have a back loaded contract (whether when he signed or if he agreed to restructuring his contract during) -which was the impression I got from last year's discussion, though I doubt the exact verbiage of the contract is public for us-, I could be as legalistically stubborn "well he signed a deal of his own free will... Does he want to play football or not ?"
We have very binary discussions, as fans, about this whole business when in fact there's plenty of money not accounted for in reported transfer fees : wages, signing bonus, agent's commission, plenty of money to grease the cogs. I'd imagine those sort of deals are pretty common place while rarely reported in details. Wages get subsidized on many loans. A player being bought out of his contract to release him is not uncommon.
By far the best financial solution for United is to transfer him off for some compensation. This can only be done with his approval. Allegedly he'll only agree if he's made whole for the earnings (fully or partially) he'll get over the two next years if he stay put. I can't really blame him for leveraging his position. I don't know if I agree on the rationale of expected earnings either, but I don't think the train of thought is completely alien or unreasonable either.