Interest money

Accumulating interest money with the intention of using it later

Q: Previously I should donate any interest accrued monthly. 

My father said I rather keep it because he will be buying a big property within a year or two, and we can use the interest for paying the property taxes, municipal rates, etc. which are exorbitant. 

Would this be okay - to withhold the interest for sometime with the intention of paying taxes?

Paying interest fees with interest money

Q: Is it permissible to pay interest fees with the interest received, from and to the same party. 

E.g. A bank rewards its client with an interest % for utilising a savings account. On the other hand, the same bank charges the client an interest fee for using its credit card facility. Can the client pay the latter with the interest received? As it will be returning the same interest to the bank.