Is Zakah (obligatory charity) due on the Zakah money received by a poor person, and reached a Nissab (minimum amount liable for Zakah), and a whole lunar year had lapsed over having it in his possession?
Yes, the poor who possessed a Nissab for a whole lunar year is obliged to pay the Zakah due on that money even if it was given to him as a Zakah money in the first place. And Allah Knows Best.
What is the ruling on someone who eats or drinks thinking that the sun has set, then realizes that it has not yet set?
Whoever eats or drinks believing that the sun has set, then later discovers that it has not yet set, their fast is invalid, and they must make up that day after Ramadan. It is not permissible to break the fast before confirming sunset—either by seeing it, through personal reasoning, or by relying on the statement of someone trustworthy in their religious commitment.
Is it permissible to authorize charitable organizations to perform the Udhiyah on one`s behalf?
Praise be to Allah, and peace and blessings be upon our master, the Messenger of Allah.
It is permissible to authorize charitable organizations to perform the sacrifice (Udhiyah) on one’s behalf. However, if these organizations undertake the slaughtering for a group of people, those in charge of the slaughtering must maintain lists of the names of those who authorized them. Furthermore, at the time of slaughter, the slaughterer must intend that the specific sacrifice is on behalf of a specific person. And Allah the Almighty knows best.
Is it permissible to offer mandatory prayer while sitting?
All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions. Standing up is a pillar in the mandatory prayer and the prayer is not valid without it unless one cannot do it. As for Nafila (supererogatory prayer), one can perform it in the state of sitting even if he/she can stand, but his/her reward is half the reward of the person who prays while standing if he/sh has no legal excuse for such an act. And Allah Knows Best.