Bride's family demanding a huge mahr

Q: I have one question regarding Mahr. The family of the bride wants to put 3 million BDT in the nikah contract which is totally unpractical and unreasonable. They are saying I don't have to pay any penny but they won't allow anything less in the contract. They might be asking this as a security in case a divorce takes place! How funny. Anyway, I would like to know the ruling on this. The bride has agreed to a much lesser amount which I can afford to pay. So if I sign the paper just to satisfy their parents and then I pay to the bride according to her wish will I be debt free? Will my marriage be valid?

Changing one's dollars on the black market

Q: I am visiting Argentina for a month. I have come across that the Peso Dollar official rate and black market rate to be very different. The official rate is around 8 pesos while the black market rate is around 15 pesos. Is it permissible for me to change my dollars in the black market? Someone told me that it is not permissible because its a type of riba and cheating and going against the law of the country. Please advise.