Consider: In the beginning of the parsha (Bereishit 12:1), Hashem tells him to leave his country and father's house to another land. Avraham does so, taking Lot and Sara with him (12:4).
Hashem then tells him that he will inherit the land, but specifically his seed will inherit the land (12:7). This is in the future. Right now Avraham does not possess the land - he is passing through. To stress this, the preceding pasuk (12:6) mentions that the Canaanites were than in (and therefore in possession of) the land. וְהַכְּנַעֲנִי אָז בָּאָרֶץ. In fact, IMHO, the purpose of mentioning the Canaanites were there, on a pshat level, is to stress that he did not gain possession of the land then.
The next psukim talk about his travelling from place to place in the land, and finally, how he must leave because of the famine, and must go to Egypt (12:8-10). In Egypt, he does not have any power, and must resort to subterfuge, having Sara lie and deny they are married so that the Egyptians do not kill him (12:11-15).
Avraham then becomes a wealthy man in Egypt (12:16), and perhaps has been able to settle down, but then he is exiled from the land (12:20) when the king of Egypt discovers Sarah is actually Avraham's wife. Thus he resumes his wanderings.
He then returns to Canaan, and the pasuk stresses he is now wealthy, as was Lot (13:1-5). This did not let them settle down in peace because of a dispute between Avraham and Lot, and between their respective shepherds (13:6-12). The land was not able to bear both of them. Thus, ironically, the wealth caused at least one of them not to be able to stay in place, as well as creating discord rather than sheket.
The cause of this dispute, according to midrash rabba, is that Lot's shepherds refused to muzzle his animals as they passed through other people's fields, while Avraham's shepherds did muzzle the animals. The cause for this dispute? Lot maintained that Hashem said he would give them the land, so the land was theirs. Why should they then muzzle their animals. Avraham maintained that they did not yet have the land, but his descendants would inherit it in the future, and thus they did not currently have a right to it. In fact, we can see the source of this midrash in 13:7:
Once again we see the stress on the current inhabitants, and the midrash takes this instance as an explanation for the dispute, especially since they are juxtoposed within the same pasuk. Avraham insisted and recognized that they did not yet have the land and were living a nomadic existence.וַיְהִי-רִיב, בֵּין רֹעֵי מִקְנֵה-אַבְרָם, וּבֵין, רֹעֵי מִקְנֵה-לוֹט; וְהַכְּנַעֲנִי, וְהַפְּרִזִּי, אָז, יֹשֵׁב בָּאָרֶץ."And there was a strife between the herdmen of Abram's cattle and the herdmen of Lot's cattle. And the Canaanite and the Perizzite dwelt then in the land."
I would add that I think the reason for this instance of "the Canaanite and Preizzite dwelt then in the land" is because in the subsequent narrative (13:9) , Avraham and Lot choose the land in which they will dwell, and Avraham is again told by Hashem that the land will be his (13:17). This is just as before when we are told that the Canaanite were then in the land, when Avraham is told that his descendants will inherit the land.
Continuing the trend, Avraham's (and Lot's) quiet is shattered by the war, and Avraham must go and save Lot (perek 14). And Avraham is told (perek 15) that his descendants will first go down to a strange land and will only inherit in the future, "for the iniquity of the Amorites is not yet full."
Thus this parsha details a very nomadic, not settled life, one which will only really be settled in the future when the nation of Israel possess the land.
When was this written?
A problem arises regarding the two psukim I cited, about the Canaanite, or the Canaanite and Perrizite then being in the land. Ibn Ezra notes the issue. (See his perush on Devarim 1:5.)
At issue is, could Moshe have written וְהַכְּנַעֲנִי וְהַפְּרִזִּי אָז יֹשֵׁב בָּאָרֶץ or וְהַכְּנַעֲנִי אָז בָּאָרֶץ? For to say that they were then (אז) in the land is to imply that at the time these words are being written, the Canaanite and Perrizite were not in the land. (Further, why mention they were in the land at that time, if after all, they are still in the land now?) It seems directed at an audience living at a time in which the Canaanites were not in the land, which would be after the Israelites took control in the time of Yehoshua. So how could Moshe have written this?
To me, this is really a non-issue. Why? Because these verses serve a specific purpose in the narrative - to note that even though talk is made of inheritance of Canaan, the settlement did not really happen back then, but other local groups were in possession. The psukim note that even though Avraham seems to be possessing the land, the Canaanite and Perizzite were then in the land. Then, at the time you would think Avraham is in possession. This does not in any way imply that now, when the psukim are being spoken and written, in the time of Moshe, the Canaanite and Perizzite were not in the land. It would possibly be more problematic were these psukim mere innocent interjections of demographic facts.
Some other explanations of the אז occurred to me:
1) They were then in the land. As opposed to when? If we are thinking of a terminus ad quem (end point) for the Canaanite occupation, then may be problematic, for even in Moshe's day they were in the land. But if we think of a terminus a quo (starting point), then it is no problem at all. At some point they were not in the land. Then they settled in the land. We are told that at this point they had already entered the land when Avraham was there. (See Rashi about Canaan vs. Shem in the land and see how this can fit in.) That they are still there in Moshe's day matters not at all. (The sefer is after all called Bereishit, The Beginning, and tracks peoples and lands from the beginning of time.)
2) The direct audience is a nation of Israel about to enter the land and possess it. As such, the expectation (and prophetic knowledge) is that in the next generation and on, the Canaanite and Perizzite would not be in the land. And while the אז and the statement about the Canaanites' presence is not necessary for Moshe's generation, the Torah was expected to be read by other audiences where this would not be the case.