How will the reckoning be on the Day of Judgment if the borrower dies without completing the repayment of the amount he took from behind his guarantor, and will it be treated as a debt, or does it have another consequence?

We praise Allah for guiding you to repentance. You have done well in appreciating the right of your companion and safeguarding your religion. Hasten to pay what you can of the right, and resolve to pay the remainder when it becomes easy for you. It is best to seek forgiveness and pardon from the rightful owner. If he forgives, you are absolved; otherwise, the remainder remains your responsibility until you are able to pay it. If you die while exerting your utmost to repay, it is hoped that Allah will satisfy your opponent on the Day of Judgment, for "So fear Allah as much as you are able."