Coffers

War can be a very costly affair. In between having to pay for keep upgrades to having to buy siege golems (or taking the time to farm them) one can assume that money keeps the gears of war lubricated. And more of often than note, correct me if I am wrong but more often than not these expenditures are covered by the commanders or interested party themselves. Trebuchet blueprints don't grow on trees and losing a fully upgraded keep is 6g down the drain. Add that to the cost of siege equipment needed to retake the keep and you have quite the cost overrun.

My suggestion is somewhat like this, a bank or war chest of sorts where the war bonds are kept. This money can come from donations, contributions by guilds etc. The money can be stored in a 'guild' established specifically for that purpose, whose membership is opened only to commanders (and on the off-chance, generous donors) for the purpose of transparency, so they can see where the money is going. The guild will be helmed by someone held in high esteem chosen preferably through a vote. And if safety is an issue certain protocols can be in place such as the chosen head of this guild having to surrender something of great worth (an orange, two oranges) to caretakers of the guild (high tier commander members). In the event of a breach of trust (embezzlement) said item can be pawned off and compensation to related parties made.

The money in this 'guild' can then be requisitioned for war expenses. This is the basic idea of it there are other points, notes and stuff to it but that can be made later.