Would you have him back for 40 mill?
I would for around £30-40m if his attitude was different, but unfortunately he wouldn't be happy being a Plan B, which is all I'd have him back for. The financial outlay is pretty much risk free as you'll always be able to con a mid-table club or foreign club into paying at least the same amount for him due to his goal record.
But I think he'd be toxic at the club if he wasn't made the "main man" and I don't think he'd accept the role of "better version of Niasse" on the bench behind a proper striker.
So overall, the only way I'd have him back for £40m is if we could immediately sell him on for £60m, as there's a profit to be made by flipping him to a gullible club somewhere.