Is it permissible to set aside a portion of a partner's money without his knowledge, for his benefit and the benefit of the business, knowing that he would not set aside anything himself?

You are not permitted to take the money without the office owner's permission. What was taken must be returned, as it is an infringement upon another's property. If you are partners, a partner is not allowed to dispose of company funds through a gift or a loan without the permission of their partner, unless they have been authorized to act on their own discretion. In that case, you may retain what is beneficial for the company without your partner's knowledge.