Genesis 15:18

In that day the Lord ordained a covenant with Abram, that He would not judge therein his sons, but would deliver them from the kingdom, saying, To thy sons will I give this land, from Nilos of Mizraim unto the great river, the river Pherath,

IN THAT DAY THE ETERNAL MADE A COVENANT WITH ABRAM, SAYING. Now the Holy One, blessed be He, promised Abraham the gift of the land many times, and all of the promises served a purpose. When he originally arrived in the land, He said to him, Unto thy seed will I give this land, but He did not clarify the extent of His gift, for included in this promise is only the land where he walked, unto the place of Shechem unto the oak of Moreh., Verse 6. Afterwards, when his merits increased while in the Land, He bestowed upon him the additional promise: Lift up now thine eyes, and look from the place where thou art, northward and southward, and eastward and westward,, 13:14. meaning that He will give him all those lands in their totality, for the meaning of the expression, which thou seest,, Verse 15. is not literally “with your eyes” for the sight of a person does not extend far. Rather, it means that He will give him [land which lies in] every direction in which he looks. It may be that He miraculously showed him all the land of Israel, as was the case with Moshe Rabbeinu. He further added in this second blessing: and to thy seed forever,, Verse 15. and that his seed will increase as the dust of the earth. At the third time, He clarified to him the boundaries of the land, mentioning all the ten nations [who presently inhabited it], and in addition He made a covenant with him that sin would not cause [the annulment of the gift]. When He commanded him concerning circumcision, He told him, for a possession forever, that is to say, if they will be exiled from it they will again return and inherit it. (13:15), does not imply that if they will be exiled from the land they will return and re-inherit it. It assures legal title but not necessarily actual possession. However, the expression, for ‘a possession’ forever, does indicate that it is to be their land forever. Hence even if they are exiled they will return and possess it. He also added at that time, And I will be their G-d, meaning that He in His Glory will lead them, and they shall not be under the rule of a star or constellation or any power of the powers above, as will yet be explained in the Torah.
Now at the time of the first gift, Scripture states, Unto thy seed will I give, the verb being in a future tense, and similarly in the second time,. (Above, 13:15). because until then He had not given him the entire land, and therefore, He said to him, will I give it. But at the third time, during the covenant, He said, Unto thy seed have I given, meaning that He will make the covenant for the gift that He had already given him. Similarly, at the time of the circumcision, when He said, for a possession forever, He said to him, And I will give unto thee, in the future tense.
Rashi wrote: Unto thy seed have I given. The word of the Supreme One is as if it were already accomplished. — But there is no need for this explanation in this passage.

ביום ההוא כרת ה' את אברם בריתת, on the occasion of that vision G'd concluded a covenant with Avram to give the entire land of Canaan to his descendants. He hinted to him by means of this vision that they would inherit all of it at the conclusion of the final exile. We know this because now G'd named 10 tribes as being the ones whose lands would all become part of the territory of the Jewish people.

נתתי, a reference to what G'd had already given to Avram. When G'd makes a promise, it is as if He had already fulfilled it, as He does not renege. Hence the past tense, נתתי. The boundaries of these tribes at that time extended as far as the river Euphrates.

עד הנהר הגדול נהר פרת, our sages (quoted by Rashi in commenting on the word הגדול, "the great one," being applied to the river Euphrates), say that even though among the four rivers listed as emanating from Gan Eden there was a bigger river than Euphrates, this one is called "the great one," as it traverses some parts of the land of Israel (in the future).