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.
Cooperative insurance is based on donations and cooperation, not on commutative contract. In terms of the Fiqh framing for this issue, the insured participate in the Cooperative Insurance Fund and donate insurance premiums through cooperating among themselves. This is in order for the Fund to compensate the affected from amongst them in line with the damage that has befallen them.
In principle, the compensation should be just and in accordance with the value of the damage, and it isn`t permissible for the insurance company to withhold what is due to the participants since Allah the Almighty says {What means}: " To the Madyan people We sent Shu’aib, one of their own brethren: he said: "O my people! worship God; Ye have no other god but Him. Now hath come unto you a clear (Sign) from your Lord! Give just measure and weight, nor withhold from the people the things that are their due; and do no mischief on the earth after it has been set in order: that will be best for you, if ye have Faith." {Al-`Araf, 85}. Similarly, it isn`t permissible for the insured to claim more than they deserve since Allah the Almighty also says {What means}: " O ye who believe! Eat not up your property among yourselves in vanities: But let there be amongst you Traffic and trade by mutual good-will: Nor kill (or destroy) yourselves: for verily God hath been to you Most Merciful!" {An-Nisa`, 29}.
In principle, the damage should be estimated by experts, but in case of dispute, the court shall have the final say.
Moreover, it isn`t permissible to claim the legal interest because it is clear usury, which is forbidden to take; particularly since the concept of cooperative insurance rests on donations. And Allah the almighty knows best.