Interest money

Upholding the laws of Islam in a Muslim or non-Muslim country

Q: I would like to ask regarding فقه الأقليات. 

What is it? Could some examples be given of where it is applied? What's Mufti Saheb's stance on it? If Mufti Saheb is for it then why? If not then why? I would really appreciate a detailed answer as this is very commonly applied by Ulama here in the West and I'm not sure if what they're doing is right or not. Using this, some have permitted interest too.

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?