What is the ruling on dissolving a partnership between two parties who agreed not to dissolve it except with their mutual consent, and now one of them wants to dissolve it? And is the condition of mutual consent for dissolution considered void?

The condition not to dissolve the partnership contract absolutely or for a certain period contradicts the nature of the contract. Thus, it is a void condition, but the contract itself remains valid, because the partnership contract is originally, according to the majority of scholars, among the permissible (non-binding) contracts.