What is the religious ruling if one is unable to pay the last installment for an item purchased in installments, due to being unable to find the merchant?

The money you took remains your responsibility until you return it to its owner. Therefore, you must search for him through all modern means, such as telecommunication companies, banks, newspapers, and so on. If you do not find him after exhausting all avenues, then give the money in charity on his behalf. If he or his heirs appear later, they have the choice between ratifying the charity and its reward for him, or taking their right, in which case the reward of the charity will be yours. You must also write a will concerning this money as a precaution.