In-laws forcing the boy to issue a divorce

Q: A secret nikaah was performed, girls parents are against it especially because this makes her a second wife and because the first wife has an infant her first child. If the husband is pressured by the parents to give talaaq and he does, on whom does the Gunaah fall? The parents openly said they are answerable for their daughter to Allah and will accept the punishment. This marriage is only causing anarchy on both sides. Please advise.

Interest

Q: My father gave some money to a broker for land so some how my father can't get the land so the broker said that he will return the amount which he has took from us but after 1 year he gave some amount from the total amount. After that my father told him why don't you return all my money but he said that he couldn't give the full amount at once because he used the amount in his business. But he said (if we don't demand it) he will return our money with interest of 2.5%. So my question is, is it permissible to take the extra 2.5%?