I don't know why you have to have 25.000 stored in the backroom safe to unlock the 28'th puzzle, just that you do.
So don't empty that save, if you want to check how much is in it you have to click it and thereby empty it, which is why I suggest making a save game before checking the safe's cashflow.
You might be able to speed up the time by making Jasmine massage oil and sell it to the masseuse, since you have to buy 1 love potion for each of those.. it's a grind fest but also a sure income, which at the same time increase what's in the safe vault.