Najafi Q&A: Banks

What is the ruling on the interest that non-Islamic banks provide to those who deposit their money in, for example, a savings account?

What is the ruling on depositing cash amounts in local banks in non-Islamic countries? Should the amount be given as khums before depositing?

If a person finds an amount transferred to their bank account or what is called today a Benefit account, and they are unable to determine the source or reason for the transfer, what is the ruling on the transferred money?

I want to buy a car from the bank by renting the car? Renting a car is to pay 20% of its value without taking a loan, followed by monthly installments during the lease term. Therefore, it is a rental and not a purchase. Is this permissible?

There is a bill of riyal currency that is still used in the market, but it has become scarce in the market as it is replaced with coins. Some keep this bill by buying it as it is a rare bill and no more issued. For example, one might sell (100 riyals for 130 riyals), knowing that if these (100) were exchanged in the bank, their price would be (100) riyals, bearing in mind that recently the issuance of currency is stopped in that it has not become a rarity yet. Is it permissible for the seller or buyer to sell or buy them for an amount higher than their actual value?