All perfect praise be to Allah, The Lord of the Worlds, and may His peace and blessings be upon our Prophet Mohammad and upon all his family and companions.
A solidarity fund is a fund established by a group of people where each donates a specific amount of money on regular basis. In case of calamity, such as death and the like, a member`s family is given an agreed upon amount of money. In fact, Sharia recommends such initiatives since Allah, the Almighty, said: " Help ye one another in righteousness and piety, but help ye not one another in sin and rancor "{Al-Mai`dah/2}.
Moreover, sums received by families are donations, although they are specific, since what counts, here, is that they aren`t owned by a particular party, they aren`t a debt due to donor from fund nor are they a commutative contract between the donor and the fund. In this case, sums to which beneficiaries are entitled are donations granted by the fund and aren`t governed by rulings pertaining to bequests and inheritance. This is provided that this is stipulated by the rules and regulations of the fund and approved by all parties.
In conclusion, it is permissible to give amounts from that fund to some inheritors if approved by fund`s regulations because once paid to the fund they aren`t his(donor/subscriber) and once he died(donor/subscriber), they aren`t part of his estate; rather, they should be given in accordance with the fund`s rules and regulations. And Allah knows best.