Probably, but after his performance this season, I suspect they were planning to try and tie him down to a longer deal with better pay anyway. A couple of big jobs are likely to be available soon, including the one which would be, for him, the biggest and best he could have in club football. MMotherwell will either get to keep an excellent manager, or could claim a fat pay-off. Either way, they win.
As for his decision for next season - whatever the issue with pay, he will get to manage his club in Europe next season, something he wouldn't have had at Hearts. He's also got a good chairman at Well, again something he wouldn't have at Hearts. So many reasons to stay...