What is the validity of an agent purchasing more than half but less than the whole item when explicitly restricted from buying the whole item?

Primary text

If the principal states, 'Buy half of it for me for one hundred, and do not buy all of it,' and the agent purchases more than half but less than the whole amount for one hundred, the purchase is valid. This follows the principle established in the previous issue, as custom implies permission for purchasing any amount exceeding half. Since the agent was explicitly prohibited from buying the entirety, the transaction remains valid for the quantity between the specified half and the prohibited whole amount, based on the remaining scope of the initial authorization.