giving extra

Q: Sometimes I secretly replace my wife's money from her purse with a bigger amount. For example, if she had R50, I take the R50 for myself and replace it with a R100 without her knowing in order to surprise her. When she comes to know of a bigger amount, I lie to her and tell her it was miracle that took place from the side of Allah. It really pleases her when she hears stuff like this, miracles etc. 

My questions are:

1. Am I guilty of the sin of lying?

2. By me replacing her money with a larger amount, is this regarded as Ribaa?

Q: My question is in business if some one lends money to other company for help and that company returns money very late but returns with extra money added. Should we keep that extra money or return? Please make a point that the lender got loss in business because of the borrower returning the money late.

Q: Say, suppose, I had borrowed some money from a certain person (not specific) about few years back. When I return the money to him, do I return the exact amount or should I include some extra amount accounting for the inflation that would have occurred during the years. Would including the extra amount mean I am giving interest and would it be a haram?

Q: If a person has a debt owing to another person and they pay them back a little more than they originally owed them, e.g they owed £90 but gave them £100 back, out of nothing more than goodness, is this considered riba? Must the exact amount be paid back?