Unusual_
He got the money from Vacher/Marcel.Vacher was the Heisenberg of Fontaine and was the biggest drug lord in the nation.Since he’s dead,and Issac knows where his base of operations is,all his wealth,notes,and connections where up for grabs.Which brings in the possibility of Issac taking over the drug trade of Fontaine,since he has Vacher’s notes on how to make it.If not,then there is going to be a huge power grab that going to happen in the underworld of Fontaine and Navia is going to be in the middle of it,as well as trying to look for the now missing Marcel.Navia’s story is now going to be way different then canon now that Marcel went out of the picture the way he did.That and the effects of the primordial water won’t be reveal to all of Fontaine until much later or in a different way since it can’t happen the same way anymore(most likely time will be when the primordial sea hits Poisson).