Absolute knowledge of the unseen belongs exclusively to Allah. However, Allah may reveal certain aspects of the unseen to some of His servants by His permission, such as through true dreams, divine inspiration, and revelation to the Prophets. Therefore, seeing future events in a dream or witnessing scenes of Paradise or Hell in one's sleep does not contradict the fact that Allah alone possesses complete knowledge of the unseen, because these are matters of the unseen that Allah has chosen to disclose to some of His servants.
The basic principle established by the Qur'an and the Sunnah is that absolute knowledge of the unseen belongs to Allah alone.
Allah, the Exalted, says:
"He is the Knower of the unseen, and He does not disclose His unseen to anyone."
(Qur'an 72:26)
Allah alone possesses complete knowledge of everything that has happened, everything that will happen, and even what would have happened had circumstances been different.
Allah may choose to disclose certain matters of the unseen to some of His servants.
This does not mean that they independently possess knowledge of the unseen. Rather, they know only what Allah chooses to reveal to them.
Allah says:
"They encompass nothing of His knowledge except what He wills."
(Qur'an 2:255)
Yes. A true dream may be a means through which Allah informs someone about a future matter.
A person may see in a dream:
Then the event occurs exactly as it was seen.
For this reason, the Prophet ﷺ described the righteous dream as:
"One of the forty-six parts of Prophethood."
Because the dreamer does not acquire knowledge of the unseen independently.
Rather, Allah has chosen to show him a small portion of it.
Just as Allah informed His Prophets about future events through revelation, He may also inform some of His righteous servants about certain matters through true dreams.
Nevertheless, complete knowledge of the unseen remains exclusively with Allah.
The scholars have mentioned several means, including:
This is the highest and greatest means of receiving knowledge of the unseen.
These are among the glad tidings that a believer may see or that may be seen concerning him.
As occurred with some of Allah's righteous servants, the most well-known example being 'Umar ibn al-Khattab (may Allah be pleased with him).
The angels convey only those matters of the unseen that Allah permits them to reveal.
The answer is: No.
Dreams are of three types:
Therefore, not everything a person sees while asleep is a true dream or a revelation about the future.
And Allah knows best.