All perfect praise be to Allah the Lord of the Worlds. May His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
If the situation is as described in the above question, then the sale has been concluded: the father sold the flat to his son, the son paid half of the price in cash and half in bills and the flat was registered in the son`s name. Therefore, it isn`t permissible for the rest of the heirs to object to this sale and everything that the father left, including the bills, is part of the estate, which is the right of all the heirs.
We advise the heirs to fear Allah and adhere to the rulings of Sharia as regards the division of the inheritance since worldly life doesn`t suffice from the Hereafter. The evidence on this is that Allah the Almighty Says {What means}: "And fear the Day when ye shall be brought back to God. Then shall every soul be paid what it earned, and none shall be dealt with unjustly." {Al-Baqarah, 281}. And Allah The Almighty Knows Best