What is the ruling of Islamic law regarding a dispute between partners over an unexpected commercial loss, given that one of the partners demands a full refund of his capital after incurring a loss due to the seller's fraud?

The partner's statement (that he wants to withdraw from the game) means the dissolution of the partnership. A partnership is a permissible contract that either partner can dissolve. If one of the partners wishes to dissolve the partnership, he bears the losses incurred by the partnership according to his share and receives a portion of the profits also according to his share. "Losses are proportional to their capital, and profits are proportional to their capital." And "Losses are proportional to the two assets." So, loss is the decrease in capital and is distributed among the partners according to their shares. Therefore, the partner's demand for all the money is unfair. He has the right to the remaining percentage of his share after deducting his portion of the loss, provided that the other partner has not been negligent with the money.