Barca would happily let him go in my opinion. He’s been nothing but a liability for them and it’s clear they’re not handling him well either.
Even if he does regain his fitness, he’d still be low in the pecking order. If Messi Suarez Griezzy are fit they all play. He’d also then have Fati to compete with.
Barca will most likely use this opportunity to trim their wage bill. Also, even if Barca did want to sell him, no one would buy him in his current situation. Not even us. A loan would be compulsory for Dembele to prove his worth. This strategy would also be more beneficial for Barca in my opinion. If they try to sell him now they know nobody will offer north of €50m approximately. But if he does prove his worth, he’ll obviously fetch a lot more and rightly so. If nobody’s willing to pay that they can just bring him back and by next summer Suarez will most likely be gone. Dembele Griezzy Messi would be a decent line up for them. Point is, if Barca did want to sell him, they’d want a substantial return on their initial investment when they bought Dembele and that won’t happen this summer for sure. I may be wording it wrong but I mean, they’ll want at least half or so back on the 150m just like you said. And not 50m.
As for his ‘serious attitude problems’, I think he’s past that phase now. Young players go through it but he obviously won’t stay immature forever. Pogba & Martial along with the young squad will have a positive effect on him.