Is it permissible for charities to take a portion of the Zakat funds designated for the poor and needy to spend on dawah (proselytizing) and its activities, or on sponsoring a student of knowledge or some preachers?

The fundamental principle is that charitable organizations act as agents, and thus, they are not permitted to spend funds except in ways specified by the donor or grantor. If an agent contravenes the instructions of their principal, their action is suspended pending the principal's ratification. If the principal does not ratify, the agent is liable for restitution. Encourage people to donate to callers to Islam and students of knowledge due to the virtue of such an act.