Saved up disability grant

Q: I would be grateful if you can answer the following query for me.

If a man was receiving disability benefit from the government and over the years some was spent and some was left as savings, what should be done with this money from an Islamic perspective.

  1. Once he has received this money is he the sole owner and at liberty to spend as he wills. e.g. Can he go hajj with this money he saved?
  2. Does he have to pay zakat on this money after a year if there is any surplus of this money?
  3. If he was to die and he left a large amount of money then what should the children's of this man do with this money?
  4. Who has more rights and responsibility over this money the sons or daughters?
  5. If a son found out that there is a possibility of that money being hoarded after the demise of this man would it be wrong to inquire about it from the people concerned?