Not sure about cash, but I find that if I want to restock/refresh a vendors inventory, I quicksave in front of them, exit the game. Load the game and then load the quicksave, their inventory will be updated. Exiting out of the game completely is only needed the first time after you have quicksaved in front of them, after that you can quicksave, exit back to the main menu, reload the save and again their inventory will have changed. Rinse and Repeat. If you load your save in a different area from the vendor (even if you have exited the game) and transition into their area to visit them, their inventory will not have changed.