The pension fund paid out a large sum of money to the estate of the deceased.

Answer

If the pension fund was a compulsory one, not a voluntary one, then the entire amount received will be the property of the deceased and subject to the law of Inheritance.

If the pension fund was a voluntary one in which the worker had entered of his own free will, then only the amount which he had actually paid towards the fund will be his property. The excess will be riba which has to be given to charity without niyyat of thawaab.