What should I do about a machine I sold to an unknown person four years ago for 15,0 DZD, where he paid 13,0 DZD and 2,0 DZD remained, and he promised to take delivery and pay the rest but he broke his promise, and the machine is now taking up a large space at my father's house? Should I sell it, bearing the selling costs and losing part of its price, then bear the remaining amount and deliver it to the buyer? Or what is the Sharia-compliant solution?

Firstly: If a person buys an item and is able to take possession of it but does not, then it is at his own risk. The seller is not responsible for it unless there is an act of aggression or negligence on his part.

Secondly: As long as you are able to communicate with the buyer, it is best to deliver the machine to him. If you cannot do so or it is difficult for you, then contact him and inform him that you will sell it, take your due, and send him the remainder. If he gives permission, then that is good. Otherwise, you are permitted to sell it after a period of time, with witnesses to that effect, and the remainder is held for him. If his news is cut off and he cannot be reached, you may sell the machine, take your money, and give the remainder in charity on his behalf.