Create a "buy fund". The lord puts the gold in it upfront, and every tax collection, it sees if the gold left in the fund is lower than what the lord put in. If it is, it deduces a portion of the lord's income and puts it to the buy fund.
Result: Auto-buy, and the gold comes directly from the lord's pockets, and out of already-collected gold so no possible negative balances. Prefferably, but not necessary: allow the lord to modify the fund size by taking out or adding gold into it, make him able to set a maximum per tax (in $ and or %) income transfer (so that he's sure to have some left for weekly expenses like troops).
And why a meagre 14 day limit? Why not "1 season", or "1 year"?
