“And he shall not increase for himself silver and gold… so that his heart is not raised above his brothers!” (Devarim 17:17)
It is a negative commandment that the king should not have too much money. The practical explanation of this is that he should not have in his hand money more than he needs for his chariot and personal servants, and about this is says “And he shall not increase for himself silver and gold!”.
Amongst the roots of this commandment is that which id explicit in the verse itself when it says “so that his heart is not raised above his brothers!”.
Amongst the laws of this commandment are that which Chazal say that it is only for personal use that he must not have an abundance of money, but for Klal Yisrael and their benefit he may have an abundance of money. This includes money to be used for guarding Klal Yisrael and their towns and places from enemies. A king who transgresses this and sets his sights on filling his coffers only to fulfil his own wishes, and not to guard Klal Yisrael and their towns has transgressed this commandment and his punishment is very great as all of the people are dependent on the king, and he must make sure to put all of his powers and aims to look after the good of his people and not look after his own honor or the fulfilment of his desires.