If he stays on the same terms that he's on now, then they have to pay him the 17million on top of whatever other wages he gets. As you say they can't force him to reduce his wage and bonus rights, so yes he's likely to stay unless Barcelona give into his demands.
But then it goes to what Barcelona are more likely to do. Either, keep a player they don't want/need, a player they've spent the summer trying to force out and make unpopular with the fans, on a prohibitively expensive contract... or bite the bullet pay the extra money they tried to swindle him out of and take the transfer fee and money saved on the rest of his wages. If they were in a financial position to eat it to prove a point they might but right now they have very little real choice. They will either give into his demands or a compromise will be reached.
Barcelona are trying to bluff at poker when everyone knows their hand.