Assalamualaikum, is it haram to say that a non muslim or disbeliever will go to jahannam. I am pretty sure it's clearly written that a person who dies while not believing in Allah or associating partners with Allah will go to jahannam, so it's clearly writen there but I heard a story from a Mufti that, there was once 2 muslims, one was a person who would always try to do good deeds and other one used to do bad deeds, the good deed one said to the other guy to stop doing this sins, but that bad deed guy didn't stop and then the good deeds guy said that, he will go to jahannam, when they both died, Allah put the guy who did good deed in jahannam and one with bad deed in jannah, the reason why he put that good deed one in jahannam because in a way, he told that he can decide whoever goes to Jannah and jahannam and not Allah, so if I someone says something like this to a disbeliever and not a believer like in this story, will it still be a sin?
This is the hadith in Sunan Abi Dawud that mentions your story. I will not quote it here because it is lengthy.
The hadith mentions that the man who did good deeds said to the man who did evil deeds:
I swear by Allah, Allah will not forgive you, nor will he admit you to Paradise.
Allah took great offence to this claim of his.
Allah said to him on Qiyamah:
Had you knowledge about Me or had you power over that which I had in My hand?
Meaning, why did you assume I would not forgive the sinner and have mercy on him? How did you promise about the future when you do not know it?
At this point, it is important to note that this hadith is not about a believer and disbeliever. It is about a man who did good deeds and a man who did evil deeds, obviously both being believers.
We know that believers may be forgiven by Allah regardless of the amount of his sins on the Day of Judgement. Not only that, we know Allah can help believers and disbelievers repent later on in his life. For both of these reasons, it would be wrong and evil to claim someone will never be forgiven or never enter Jannah when they are a believer or when they are still alive and have time to repent.
This is regarding speaking about specific people by name. It is certainly allowed to say, in general, sinners will be punished.
As for after a disbeliever has died, is it allowed to say he will not be forgiven or will not enter Paradise?
There are different opinions on this topic, and the best opinion is that we should not say it about specific people either because it is possible they might have accepted Islam near the end and you don't even know about it.
Others have said: We simply judge by the apparent, and if a person apparently died a disbeliever, we assume he will go to Hell, and there is nothing wrong with that assumption even if it may be wrong in some strange cases.
But, it is definitely correct to say: A disbeliever who died in that state will definitely be punished in Hell and will never enter Jannah, because this is a promise from Allah in the Quran. But, we should try not apply this to specific people until we have evidence from Allah like for Pharaoh or Abu Lahab.
you can check this discussion about this issue https://www.youtube.com/watch?v=fqZpNj76HtI