What is the religious ruling on a partner demanding a guarantee of the principal capital, or a portion thereof, when a joint commercial project ceases operation and there are financial losses?

What occurred between you is a partnership (sharikah) and a commenda (mudarabah); the capital is from both of you, and the work is from you. This is valid. What your partner demands regarding the guarantee of the capital has no basis for him; the loss is borne by each of you according to your share in the capital. And since the work is solely from you, you also lose your effort. There is no guarantee upon you for the loss unless you were negligent or transgressed. The existence of a trust receipt with him does not permit him to demand the entire capital from you, but only what remains of it.