Q 1: What is the ruling on offering Salah (Prayer) behind a sinner, such as a person who shaves his beard or smokes?
A: Scholars differed on this issue: some held that Salah behind a sinner is not valid for his weak religiosity and untrustworthiness while many scholars saw that it is valid. However, the rulers should not appoint sinners as Imams (those who leads congregational Prayer) while there are others to undertake this job. This is the right thing to do. This sinner is a Muslim and knows that Salah is obligatory, so the Salah of those behind him is valid. The supporting evidence on this is what was authentically established in the Sahih (authentic) Hadith that the Prophet (peace be upon him) said about offering Salah behind corrupt leaders, They will lead you in Salah; if they offer it properly, the reward is for both of you; and if they offer it improperly, the reward is for you and the sin is for them. Other Hadiths that stress the same meaning were also reported from him (peace be upon him). Some of the Sahabah (Companions of the Prophet) offered Salah behind AlHajjaj, who was one of the most corrupt people of his time. The Mu'mins (believers) should offer Salah in congregation, even if the Imam is a corrupt person. However, if they can offer Salah behind a just Imam, it will be better for their religion.
Q 2: What is the ruling on exchanging silver riyals for paper riyals at different rates?
A: There is some discrepancy concerning this matter. Some modern scholars confirmed the permissibility of such dealings, because paper money is different from silver. Others said that it is prohibited (Haram), because paper money is a currency that is circulated among people that replaced silver; so it comes under the same rulings. As for my own view, at present I am not sure of either opinion, but I recommend being on the safe side, so it is better not to do that, because the Prophet (peace be upon him) said, Leave what causes you doubt and turn to what does not cause you doubt. The Prophet (may Allah's Peace and Blessings be upon him) also says: Whoever guards himself against doubtful things keeps his religion and honor blameless... The Prophet (may Allah's Peace and Blessings be upon him) also says: Virtue is a good moral, and sin is that which is abrasive to the soul, and that you do not like people to know about.
Therefore, in order to be on the safe side it is better to sell silver coins for something else, such as gold, then buy paper money with that. If the one who has paper money wants to buy silver coins, he should sell the paper for gold and so on, then use that to buy the silver coins he wants.