If they terminate the contract, and he's no longer playing for them, then they can surely write off the £35m immediately. I.e if he's not there, to play, and he's not on their registration, he has no "worth", and then their balance sheet will effectively show £0 for him, as he won't be one it, so basically the depreciation figure goes out the window. The "depreciation" is just a basic valuation method, for "normal" circumstances, which the tax man lets them get away with, for assets you're actually keeping, until you sell them, but if they go for zero, then the worth is zero, so it all gets written off. Surely FFP will see that they didn't want to shell out all that cash, for zero return, it's not like a company buying kit early in a good year, to dodge a corporation tax bill, when they know they might need the kit next year etc.
They might not have written off Greenwood, they're probably playing it by ear in the off chance there's going to be a positive development, but we all know it's not coming. He was effectively guilty in all but conviction, nothing is going to counterbalance that and they know if they could have had him ever play in a Man U shirt again, then the best chance to do it was this year and just get the carnage out of the way. They know the fans would never really accept him back, never mind the rest of English football. He can get away with it abroad, as it's more out of sight, out of mind, but that's poor form.