|כג וַיֹּאמֶר יְהוָה לָהּ, שְׁנֵי גֹיִים בְּבִטְנֵךְ, וּשְׁנֵי לְאֻמִּים, מִמֵּעַיִךְ יִפָּרֵדוּ; וּלְאֹם מִלְאֹם יֶאֱמָץ, וְרַב יַעֲבֹד צָעִיר.||23 And the LORD said unto her: Two nations are in thy womb, and two peoples shall be separated from thy bowels; and the one people shall be stronger than the other people; and the elder shall serve the younger.|
But the ending of it, וְרַב יַעֲבֹד צָעִיר, is ambiguous in three separate ways, all of which confuse us as to whether Esav's descendants are to serve Yaakov's or vice versa.
It is ambiguous due to the consonantal text. It is written chaser, despite there being a cholam sound at the end. And so in Bereishit Rabba:
ורב יעבוד צעיראמר רבי הונא:אם זכה יעבוד ואם לאו ייעבד.
Thus, it could either be the niphal or the kal. If they merit, the older, Esav "will serve" the younger; and if not, they "will be served". And different nikkud can fit into this consonantal text.
It is also ambiguous because of the meaning of the lexical items. Indeed, in Akkadian, there are the cognates rabu and tzechru, where these are roles for the one with the firstborn right and the one without the firstborn right, where these rights can be transferred. If so, who is the rabbu and who is the tzechru? Who will serve whom? It is left unclear, due to the selling of the birthright later on. Either Yaakov or Esav can be the firstborn here.
It is also syntactically ambiguous, due to the lack of the et object marker. We would normally expect verav yaavod et hatzair. Without the et, in theory, either one might be the subject, and either one might be the object. I've seen this argued elsewhere, but I most recently saw this theory put forth in Ibn Caspi (assuming I understand him correctly). He writes:
ורב יעבוד צעיר . אלו אמר אתצעיר או את רב היה מבואר המבין יותר, אבל הפליג נותן התורהלהניה זה בלי הכרע, בעבור שפעם יגבר עשו, ופעם יגבר יעקב כמו•שסיים יצחק ברבותיו, ולכן מבואר כי ורב וכן הפכו משותף ודי בזה:
That is, the et would have made it clearer which was the object and which was the subject, but Hakadosh Baruch Hu deliberately made it ambiguous, so that either one can be the rav of the tzair, for reasons that are clear later on in the parsha, where we see from Yizchak's blessings that at different times one prevails over the other.
Now, there is a theory which has gained popularity in general in recent years, called multi-valence. How can so many different meforshim all see different interpretations in the text? And we have an inclination to say elu veElu, even though it might not be justified here. The answer is that all of the meanings were intended. How so? The Author intended to be ambiguous, in order to convey all of the meanings which were deduced by later readers. So which is true? All of them, and none of them. None of them in exclusion, but rather there is tension in the text and the ambiguity is the message.
A classic example of this is when Moshe goes out to see his brothers. Is Moshe an Egyptian (as described in the next perek)? Or is he a Hebrew?
It is not made clear, and different readers might say that
Similarly over here, by the prophecy directed towards Rivkah, the ambiguity is deliberate. And this is what Ibn Caspi is saying. And later events (such as the sale of the birthright), and later conduct, might help resolve the ambiguity. Or as Ibn Caspi is saying, both are intended.
This might be good peshat. But on the other hand, I would point out that it is also quite possible and plausible that no ambiguity was ever intended. In terms of spelling, Ibn Ezra would tell you that this is the regular spelling, and if occassionally you have malei, this is just because spelling is not entirely standardized. If in terms of the rav and tzair, perhaps this just means older and younger, and it was clear that Esav was older. Or that this was a prediction that the older would sell his birthright and thus have to serve under the technically slightly younger brother. And in terms of the missing et, sure it would have been slightly clearer with the object marker present, but it would have been less poetic. This was in keeping with the style of Biblical poetry. And perhaps since the more obvious interpretation is that the older will serve the younger, it was not written with Ibn Caspi's stretching and reinterpreting in mind.
In sum, I see these ambiguities, and they might be deliberate. On the other hand, they can all be explained in other ways, so maybe not.