Problem with lending people money

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Last monday, I lended RM20 to a not so close roommate. He promise that he will return the money to me on night time. But he did not. The next time I met him was when he is cooking. I have to ask for that money then he pay me RM10 and the balance he said will return to me when he done with his cooking. However, no story after that. The next day I met him and he said he did not have money. I asked for his phone number but he just gave me one since he have 2 phones. He claimed that the other phone does not have sim card so I asked him if there is no sim card yet you can whatsapp. He just kept quiet. I wanted his passport but he did not want to gave me. I asked him whether you have anything to compensate me. He gave me a gold necklace with receipt. I told him it might be fake and I gave back to him. I told him that I will be moving out from my rented unit (it is true) and he promised to pay me the balance on Monday night. Should I told other roommate that they should avoid this person? How about new person staying in? Before this, I also lend him RM10 for the first time. He paid me few days late. Am I too kind or I am just stupid?