Rabbeinu Bachayei
The words עד הנה mean: “until the measure of their sin is full.” Actually, the Torah should have written: יבואו הנה “they will come here.” The reason the Torah chose to write ישובו הנה ‘they will return here,” is to point out that they will be considered as if they had already been settled in the land of Canaan previously seeing that Avrohom, Yitzchak, and Yaakov and his children had lived in Canaan before descending to Egypt. Remember that Yitzchak had never left the land of Canaan. Concerning Yaakov, the Torah had written וישב יעקב בארץ “Yaakov settled in the land.” This is why now the Torah wrote that the fourth generation would return “here.” They would return to what had already been their land.
Another message contained in these words may be an allusion to the sanctity of the land, the land in which there is found the “gateway to heaven” (Genesis 28, 17). This means that the Sanctuary of the terrestrial regions is on the soil of the land of Canaan. This is the point from which the souls ascend to the celestial regions. This is also where they enter the terrestrial regions. It is appropriate therefore to describe the land of Canaan as the place to which the souls return. The righteous and pious people therefore experience an urge to die near that location in order for their souls to immediately find the path to the celestial regions.
Radak
כי לא שלם עון האמורי עד הנה , the time when the Emorite is due for punishment will not be until then. G'd is very patient with sinners, giving them a great deal of time to mend their ways. The punishment for sin is called עון, as we know already from Kayin (Genesis 4,13) who said of the punishment for his having murdered Hevel, his brother, גדול עוני מנשוא, "my punishment is too great for me to endure." We also find that the word חטאת is used both as sin and as atonement for sin, just as the word עון sometimes means "sin," and sometimes "punishment." Compare Zecharyah 14,19 זאת תהיה חטאת מצרים. The reason the Torah singled out the tribe of the Emorite among all the Canaanite tribes is because it was the strongest of those seven tribes. The prophet Amos 2,9 also quotes G'd of having destroyed the Emorite on account of the Jewish people, although the same fate overtook the other Canaanite tribes also.
