Is it permissible for me to say to someone who has died, "Smell the fragrance of Paradise"?

And he is a Muslim. Is this overstepping Allah's authority? Yes, it is overstepping Allah's authority. Paradise is not in your hands.

Paradise is in the hands of Allah, the Exalted. Neither you nor I own it. It is not permissible to say to anyone, "You will not enter Paradise." This is the same as what we mentioned before: "Allah will not forgive."

That you claim authority over Allah.