Answer
Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
The debt on the house that your father has to pay does not affect the Zakah he has to pay if he possesses the nisab, as this debt will be paid from future income.
In his response to your question, Prof. Dr. Monzer Kahf, Professor of Islamic Finance and Economics at Qatar Faculty of Islamic Studies, states:
The debt on your father is against his future income. It is not going to be paid from his present funds and accounts.
Let me assume that your father has a bank account, some stocks and shares and business the total of all of which on the day of Zakah is more than the amount of nisab, Zakah is due on him on all this balance as long as the debt is not going to be paid out of these items.
Debts on a house are always to be paid from future income not present items.
Add to that the fact that your mother is, according to the Shariah, financially independent from her husband. She is required to pay Zakah on her own property while he may be considered poor.
For the lady who has money in the bank and lets her brother use sometimes, she is definitely required to pay Zakah on the balance of total money, the bank accounts and debts on her brother.
Allah Almighty knows best.
Editor’s note: This fatwa is from Ask the Scholar’s archive and was originally published at an earlier date.