Assalmu Alaykum. My question is that If I want to give some one a thing like my phone to my brother and in my mind I make an intention that it is now his but I don't tell him and he thinks of it as my property.Under such a circumstance islamically whose property is the phone mine or my brothers.
You have not physically given him the gift, but merely made an intention to do so. The property still belongs to you as you have the freedom of changing your intention.
Once you give him the object, it will be considered his.