Let's say there is a person who has three phones, which they don't all need at the moment. And one of the person's coworkers asks them if they can use one of the person's phones because their phone went dead, but the person refuses to lend them a phone. What adjective would you use to describe a person who doesn't share their stuff when they don't need it? Can I say they are greedy?
You can say they are possessive. The Lexico entry has
1.1 Showing an unwillingness to share one's possessions.
young children are proud and possessive of their own property
I would say the word greedy is incorrect here. A greedy person would obtain more phones than they actually need.