Yes it's a difficult situation but I'm not sure the easy statements of Palace being in the wrong are helpful. The lad signed a long term deal with Palace, who are now in a very strong negotiating position as a result and are exerting it accordingly. He was happy enough to sign for the extra money, so unless some sort of gentleman agreement accompanied it (or a formal clause) Palace are well within their rights.
The difficulty is that his value to Palace and his value to other teams is greatly different. I don't think I'm being disrespectful to suggest he was probably the key difference between survival and relegation over the past couple of seasons. Maybe he will be again next season? So his value is enormous, before you even get into the sentimentality of him being a captain and local lad etc.
That is weighed again his value to others being different. For Everton, he would likely be our best player, or very close, but probably at a similar level to Sigurdsson, Digne, Richarlinson etc. While there has been a lot of talk of bids coming in, nothing has come from Dortmund, Arsenal bid around the mid 40's before moving on and paying 70+ on another player. Everton's bid of around the mid 50's at present is the best deal. I'd go as far as to say we are almost bidding against ourselves.
All of the above is exacerbated by what seems an unusually large sell on clause in the deal. I do feel for Palace though, I've seen somewhere the American owners are getting a bit twitchy. A deal of say 60 million, cut off the sell on fee leaves you with around £48 million and 3 days to spend it. You could sign say Tosun and McCarthy which would be the best part of £25 million and 1 other. I can see why for Palace, when you are talking the difference between relegation or a mid table finish the option of keeping Zaha seems preferential.
A lot will come down to the lad and how the club know him. How will he perform if he is kept there if there was some agreement he could go? Thats the big question really. We had similar with Lookman last season, he had a negligible impact and was sold at lower than we could have got last summer. It's wrong to say he will completely throw toys out of the pram, but will he go the extra mile as he used too? If he doesn't what might his impact be in the dressing room, if things get difficult? These are all factors that only Palace can answer.
I don't really envy Palace at all in this situation. Seems like lots of things are going on. My gut is you might buckle next week, as I sense Hogdson may struggle to manage Zaha if he is unhappy at his treatment.