The repayment of the debt can never be fully guaranteed. This is where the collateral comes. The Lender might want 2x collateral / debt ratio in order to lend its funds. This is why the debtor should want to return his debt because otherwise he will not get his collateral back which is usually worth more than the debt itself. Also there will be an option to repay partly fiat/collateral. But once again, the Lenders will decide the terms of the contract. We are just giving them the field where they can do this.