The Torah in Parashat Shoftim commands that a king is not permitted to have a large amount of silver or gold – “Ve’chesef Ve’zahab Lo Yarbeh Lo Me’od” (Debarim 17:17). The king is permitted to collect as much money as needed for the nation, such as to maintain an army and for other matters of public concern. This Biblical command forbids the king from bringing money into his personal coffers for his own luxury and enjoyment, beyond that which he legitimately needs.
The Sefer Ha’hinuch warns that a king who transgresses this command will be severely punished, though the he does not mention that the king is liable to Malkut. The Rambam, by contrast, maintains that a king would be liable to Malkut for accumulating extra wealth, as this violation is committed by performing an action. The commentators explain that the Sefer Ha’hinuch likely felt that Malkut cannot be administered for this violation because the witnesses cannot know the king’s intent when he amasses the extra wealth. Anytime the king collects money, it is unknown whether he does so for the needs of the people, or for his own luxury. Therefore, the witnesses cannot definitively testify to the violation of this prohibition, and for this reason, perhaps, the Sefer Ha’hinuch felt that the king does not receive Malkut for transgressing this command.