Is the tenant entitled to demand the解除 of the mortgage and the cancellation of the lease agreement after he defaulted on paying the value of a car he obtained from his partner, who stipulated full ownership of the site in case of non-payment, knowing that he incurred significant losses as a result?

You are permitted to request the creditor to release the mortgage if he agrees, and it is permissible to expedite the repayment of the debt to release the mortgage. As for requesting the reduction of part of the debt in exchange for expedited payment, scholars have differed on its permissibility. The preponderant opinion according to the Permanent Committee for Issuing Fatwas, which is also the view of Ibn Taymiyyah and Ibn al-Qayyim, is its permissibility.