Will the sponsor of a preacher, through whom a disbeliever embraces Islam, receive the reward for guiding this disbeliever? And does the ḥadīth: "For Allah to guide a single man through you is better for you than red camels" apply to him?

Your sponsorship of the preacher and your encouragement for him to engage in Da‘wah (calling to Islam), and a disbeliever embracing Islam as a result, holds immense reward. This is based on the Prophet’s (peace and blessings be upon him) saying: “For Allah to guide a single man through you is better for you than red camels,” and his saying: “Whoever calls to guidance will have a reward equal to the rewards of those who follow him,” and “Whoever introduces a good practice in Islam will have its reward and the reward of those who act upon it.” Imam An-Nawawi clarified that these hadiths encourage the initiation of good deeds and prohibit evil ones, and that whoever calls to guidance or misguidance, whether he originated it or was preceded in it, will bear the reward or sin of his followers. Calling to Allah is among the noblest deeds; it is the function of the prophets, and it requires sincere intention, as Allah the Almighty says: {Say, "This is my way; I invite to Allah with insight"} and {And who is better in speech than one who invites to Allah and does righteousness}.