Hi Rabbi. I tried looking this up but could not find a source. If I borrowed a sweater from a friend or relative and forgot to return it and many years go by and they never have asked me for it back, Can I assume they have given up on it and keep it? What happened if I heard them explicitly give up hope of its return. Thanks
An item that you borrowed and never returned in a way is considered like you stole it, and has to be given back, no matter how long afterwards. Additionally the idea of yiush, is only when the item was outside of the person’s possession, such as lost, and then he was miyayesh. In your case the owner gave it to you and you are no less than a shomer, and it has to be returned. If the owner tells you that you can keep it, then you can.