“This is the land about which I (G-d) made an oath to Avraham, Yitzchok and Yaakov, saying, ‘I will give it to your descendants’” (D’varim 34:4). The Talmud (B’rachos 18b-19a), apparently trying to explain why the word “saying” is needed, tells us that G-d was asking Moshe to tell our forefathers that He had fulfilled the oath he had made to them centuries earlier. These instructions to Moshe, the Talmud says, prove (1) that the deceased converse with each other, as Moshe fulfilled G-d’s request and spoke to our forefathers after he died, and (2) that the dead are aware of what is happening to the living, as if they were totally removed from our world, Moshe relaying this information wouldn’t register (see Rashi).
In his commentary on Chumash, Rashi references the Talmud’s explanation of the word “saying,” but doesn’t include the context. It is clear from the way he worded it, though, that he understands the information Moshe was to relay to be based on what he saw while he was alive, which raises the issue of how he could tell our forefathers that G-d had fulfilled his oath if the Promised Land wasn’t conquered by the Children of Israel until after Moshe’s death (seehttp://rabbidmk.wordpress.com/2014/09/18/parashas-nitzavim-vayeilech-5774/). The Talmud must have also assumed that Moshe was supposed to relay information gathered while he was alive, since it tried to prove that the dead are aware of what happens in our world from our forefathers being aware of what happens, not from Moshe being aware of what happened after he died (and then reporting it to them). Nevertheless, once it was established that the dead are aware of what happens to us, it is possible that G-d wanted Moshe to tell our forefathers what happened after he died, after the land was actually given to the nation.
As far as why Moshe would be asked to tell them something that happened after he died rather than our forefathers finding about it on their own, the Talmud asks this question, answering that the reason G-d asked Moshe to inform them of something they already knew was “so that they could give Moshe credit for it.” What this means is unclear. Or Hachayim (D’varim 34:4) suggests it means so they can be thankful that Moshe worked so hard on their behalf to bring the nation to the Promised Land. I’m not sure why they wouldn’t have already realized how much Moshe had done throughout the 40+ years (starting with the plagues in Egypt), or why they wouldn’t have already been grateful for all he had done even if he didn’t tell them information they already knew. Perhaps Moshe wouldn’t have bothered them, nor would they have disturbed him, had G-d not asked Moshe to speak to them, with the information itself not being the focus, but the means through which our forefathers could express their thanks. It is also possible that G-d wanted to give Moshe one final mission, one that could only be accomplished by dying and speaking to our forefathers on the “other side,” in order to help ease Moshe’s transition from a life dedicated to doing things for G-d and His people to an existence where he could no longer do that (see Ohav Yisroel, quoted by Bais Yosef on B’rachos 19a).
One thing that seems puzzling about the Talmudic discussion regarding whether or not the dead are aware of what occurs in our world is the omission of an earlier Talmudic explanation (B’rachos 9a-9b) of G-d’s request of Moshe to have the nation ask the Egyptians for expensive items before they left, “so that [Avraham] won’t say that G-d kept His word regarding the slavery and the oppression but didn’t keep His word about leaving with a great amount of possessions.” If Avraham wasn’t aware of what was happening, this wouldn’t be an issue, yet the Talmud doesn’t quote this verse (or this explanation of the verse) to prove that Avraham must have been aware of what was happening in Egypt. [As a matter of fact, the “great amount of possessions” actually referred to the spoils gathered at the splitting of the sea a week later, not the gold and silver items they took with them out of Egypt (see Vilna Gaon on B’rachos 9), so Avraham would have had to be so on top of things that G-d was concerned he would be worried about that promise for the week in-between!]
It should be noted that the source of the Talmudic teaching regarding the gold and silver taken out of Egypt was “the study hall of Rabbi Yannai” and the source of the Talmudic teaching regarding Moshe being asked to tell our forefathers that G-d had fulfilled His oath about the Promised Land was Rabbi Yonasan. Since Rabbi Yannai and Rabbi Yonasan where contemporaries (see Jerusalem Talmud, Kidushin 1:7), a teaching coming out of “the school of Rabbi Yanai” was later that Rabbi Yonasan’s teaching. With the Talmud’s focus being specifically on Rabbi Yonasan’s opinion regarding what the dead are aware of (see Maharsha), something taught afterwards could not be included in this discussion. Besides, it may have only been after it was established that the dead are aware of what goes on in this world that leaving Egypt with expensive items could be explained in that context. Nevertheless, there is another possibility.
It is quite unlikely that Avraham would question G-d fulfilling His promise. And if it was G-d’s promise that was being questioned, there is a larger issue than just Avraham questioning it, as G-d would have to keep His word whether or not it bothered Avraham (or anyone else). Why attribute the need to fulfill the promise to how others would perceive things rather than to the need to fulfill the promise itself? I would therefore suggest that although the Talmud attributes this “concern” to Avraham, it is really a euphemism for a concern that some of his children might have had at the time of the exodus because of the promise made to Avraham. Not a concern about whether or not G-d would fulfill His promise, but whether or not the time had come for G-d to fulfill that promise.
Although Avraham was told that his descendants would be “strangers in a strange land for 400 years” (B’reishis 15:13), they were only in Egypt for 210 years (see Rashi there). Some, such as Dasan and Aviram, doubted that it was time for the redemption yet, thinking that there were still 190 years of exile left (see www.aishdas.org/ta/5765/beshalach.pdf). A large percentage of the Children of Israel didn’t think it was time to leave yet either, which is why so many died during the plague of darkness (see Rashi on Sh’mos 13:18 and Rosh on Parashas Bo). Doubts about whether it was really time for the redemption may have also entered the minds of those who did leave Egypt, especially since along with the 400 years that Avraham was told about was a promise that they would leave with great riches, and here they were, moments from what was supposed to be this redemption, and they had nothing! Getting the spoils at the sea a week later may dispel these doubts, but if they leave Egypt empty-handed, the doubts could persist until then. Therefore, G-d asked Moshe to have the nation ask the Egyptians for expensive items now, whereby they would leave with riches.
If the reason for this request was not to alleviate Avraham’s concerns, it has no bearing on the issue of whether or not the dead are aware of what is happening in this world. There was therefore no reason for the Talmud to bring it into that discussion.