What is the ruling of the Shari'ah regarding a mother's refusal to pay the remaining debt to a person who impounded her son's car for four months, based on her suspicion that he used the car during the period of impoundment?

If the debtor is procrastinating in repaying the debt despite being able, then the creditor is excused in collecting his right in this manner, and the debtor is obligated to fulfill his debt and is not permitted to make the creditor swear an oath. However, if the debtor is not procrastinating, then the creditor is not permitted to do what he did, and he is considered a usurper of the car after the agreed-upon period, and the debtor is permitted to make him swear an oath that he did not use the car.

And if the creditor refuses to swear an oath, the matter is referred to the judiciary to estimate the rental value of the car and deduct it from the debt amount, and the creditor is given the remainder, if any. There is no need to make him swear an oath, because the usurper guarantees the usufruct of the usurped item with its rental value, whether he benefited from it or not, because usufruct is a valuable asset.