1 hour ago, nameless ronin said:A lord is supposed to provide for his retainers, but that doesn't mean there's a general system with samurai putting their chop on a bill so it can be forwarded to their lord's chief of accounting. If we're dealing with functional gear, the retainer can likely requisition what he needs from craftsmen employed by their lord - those craftsmen get a wages, so no separate bill required. For other items, if the seller is local payment can be provided on delivery - that will allow the samurai to avoid being involved directly in the money exchange, since a servant will pay when accepting the goods. A lord might make an arrangement with local establishments to "comp" his retainers, or might simply own the place so his retainers get free service. In general though, retainers get a regular stipend or allowance to use as they see fit (it'd be problematic to send the proprietor of a gambling house to your lord to cover your losses, for instance) and if their duties require them to leave their lord's lands retainers will be provided with funds to deal with travel expenses. Exceptions are always possible of course, but the owner of a respectable inn in the Crab lands (probably backed by a local daimyo, so killing the servants is likely to have some consequences at least) isn't going to send a bill across the Empire to get a Phoenix courtier's lord to reimburse him for a meal and some drinks.
I can't recall ever seeing anything that says samurai should not interact with currency, by the way. Actual trade is seen as crass, but samurai can handle their zeni just fine as far as I'm aware.
TL;DR version:
Your Daimiō is not your ******* accountant. In fact he might be paying you a stipend to be his ******* accountant, and he sure as **** doesn't want to have to deal with signing off your every single expenditure of money, much less those of thousands of additional retainers.