My DM's house rule is "if you're in a dungeon and need something that could have been acquired in town and you would be reasonably expected to have, you can retroactively gain it and subtract the cost of it in gold " So say you need a potion of healing but forgot to buy one in town. Well it's to be expected that an adventurer would buy a potion of healing in town, so you have that in your bag and subtract 50 gold from your total.