To make sure that I understand your question: According to Tosfos, Duma was not yet aware of the impending death of Shmuel. If so, how did the Neshamos know that Shmuel was going to die?

To answer your question we have to first understand the meaning of the Gemara's refutation "Duma Kadim v'Omer Lehu". The Gemara means to say that even though Neshamos don't know what is happening in this world in general, they know who is about to die. The reason for this distinction is that anything pertaining to death is under Duma's jurisdiction, so he surely knows who is about to die. Since Duma resides with the Neshamos, he can tell them who is going to die. That is why they know who will die even if they have no knowledge of other things that are happening in this world.

Tosfos explains that in the case of Shmuel, Duma would not yet have been made aware of Shmuel's impending death. Therefore the Neshamos must know what is happening in this world even without being told by Duma.

In short: You asked how the Neshamos knew that Shmuel was about to die if Duma didn't know yet. The answer is that this indeed is the Gemara's proof that Neshamos are aware of anything that happens in this world, and don't need to be told by a Malach.

Shevach Pepper

The Gemara is trying to clarify whether Neshamos know everything that happens in this world or not. The Gemara asserts that they do not know what goes on in the world, and the only reason why they know who is going to die is that Dumah, who is responsible for bringing people's Neshamos to the next world, announces who is coming next.

The Gemara attempts to prove from the incident with Shmuel and his father that the dead know what is going on in this world. According to Tosfos (Rashi disagrees), the proof is that since Shmuel's father (his Neshamah, that is) knew that Shmuel was coming to die "soon," but not imminently, it could not be that he knew because Dumah told him, since Dumah only makes such announcements immediately before a death. Rather, it must be that Neshamos know everything that happens in the world.

The Gemara refutes this proof as well. Perhaps Neshamos do not know everything that happens in this world, but since Shmuel was a very important person the Neshamos were notified of his impending death so that they could make room for him.

In short: You asked why does the Gemara not say simply that Shmuel's father knew that his son, Shmuel, was about to die. The answer is that the Gemara is taking the position that Neshamos do not know what happens in this world.

Shevach Pepper