Is it considered a sin to name a newborn with a name other than "Hasan" after having prayed for him in Mecca, and does giving Hasan the title of one of the righteous (Salihin) disrespect the Master of the Youth of Paradise?

If you made a vow to name your newborn a specific name, as a vow to perform a permissible act, then scholars have differed regarding the obligation of fulfilling this vow. The preponderant view, in our opinion, is that it is not obligatory. Therefore, you are permitted to name the child any name that is Islamically permissible, and there is no sin upon you for changing your mind about the vowed name. However, it is preferable to name the child with the vowed name to avoid disagreement among scholars. But if you did not make a vow, there is no sin upon you for changing your mind about the name you intended.