Their father left behind haraam money

One of the
duties which the children owe to their father is to give the money which
he earned from selling these shares to something which will be of public
benefit, such as building roads, schools and other facilities. But it
should not be used to buy food for them or for others – to be on the
safe side – and it should not be used to build a mosque. By paying out
this money they will clear their father of sin, but this will not bring
them closer to Allaah, because Allaah is good and only accepts that which
is good.

 Whether
they pay the exact same money or an equivalent amount from their own
wealth, it is OK, because the prohibition has to do with how it was
earned, not the currency itself. If they pay an amount equal to that which
was earned, this will be sufficient.

 Ibn
al-Qayyim (may Allaah have mercy on him) said concerning a principle which
he mentioned about mixing permissible things with forbidden things: “…
there are two kinds of this. The first is when the forbidden thing is
haraam in and of itself, such as blood and urine… The second is when it
is haraam in the way in which it was earned, not because it is haraam in
and of itself, such as money that is taken by force. This second kind does
not necessitate avoiding that which is halaal or prohibiting it in
totality. If his money is mixed with haraam money, or is mostly haraam, he
should take out the haraam amount, then the rest becomes permissible for
him with no sense of it being makrooh, regardless of whether what he takes
out is the exact same money that he earned in a haraam manner or is an
equivalent amount, because its being haraam does not have to do with the
essence of the money itself; it has to do with the way in which it was
earned.