What is the ruling on using an iPhone screen that was removed from a phone locked with the iCloud system, knowing the possibility that the phone might be stolen, lost, or that its original owner forgot their passcode and sold it?

What people possess and how they dispose of it is their right. Judgments are not built upon what we do not know of hidden matters, and mere probability has no effect. The default is that what a Muslim possesses is his property if he claims it to be so. If you do not know the status of the money in someone's possession, then base your judgment on the default, for ignorance of the actual situation renders what is known as if it were unknown. This is further confirmed for the questioner by his statement that the technician is a person of faith and good character. We advise refraining from obsessions and avoiding excessive scrutiny in questioning.