Ok first you have several thing wrong in your statement, the bag is not identical it's in a corn sack in Red Dead Redemption while in American venom it's in a chest then moved by john into a leather saddle bag. Also it dose not contain the same amount as money in the caves at beaver hollow was over $40,000. With that out of the way it's made clear several times in the story that the money in blackwater was ALL the gangs money and gold from years of robbing so it'd be well over the $150,000 the took from the ferry, let's round for simplicity sake and say around $200k . Now we know that dutch was seen in TT in 1906 which is probably when he had micah reclaim the money so it'd been around a year giving micah plenty of time to spend a great deal of it by American Venom. Another thing of note is that during the gun fight with Micah, says along the lines of the blackwater money being in his cabin, but hen says "most of it" while chuckling meaning it's probably been spent somewhat. Now getting into why John has only $20kby the end, we see that there's no way John could have fit all of it in the bag so he probably only got some cash and gold and left some in the chest, then obviously he paid his existing loan but we see he builds a corn silo, buys 2-3 cows, 3 horses and most likely spent some for his wedding as well as give some to at least Charles as we would definitely need something to start a life and sadie most likely wouldn't as she appears to be independently wealthy. Anyway most of that is speculation as to why John only has 20k of what would be several millions in todays money.