Not "again" but "finally". Because God always keeps His promises,
But He also promised to give Abraham and his descendants the land between the Nile and the Euphrates for an everlasting possession (that is probably ten times the area of modern Israel).
Two things here. Genesis 15:18, does not say the Nile and God says that He gave them all the land He had promised.
Gen 15:18 In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from
the river of Egypt unto the
great river, the river Euphrates:
Whatever river is being referred to in this verse is less than the Euphrates. I don't see how anyone could say that the Nile is less than the Euphrates?
If the Nile, in Egypt, is the border of the promised land, then they were already living in the promised land when God said.....
Deu 11:10 For the land, whither thou goest in to possess it, is not as the land of Egypt, from whence ye came out, where thou sowedst thy seed, and wateredst it with thy foot, as a garden of herbs;
Deu 11:11 but the land, whither ye go over to possess it, is a land of hills and valleys, and drinketh water of the rain of heaven,
and
Exo 3:17 and I have said, I will bring you up out of the affliction of Egypt unto the land of the Canaanite, and the Hittite, and the Amorite, and the Perizzite, and the Hivite, and the Jebusite, unto a land flowing with milk and honey.
In Joshua 13, Josh is getting old and God says there is still land to be possessed.
In Deut.19 God told Moses to build three sanctuary cities, and if God gave all the land to them, they should build three more.
In Joshua 20:7-9 it tells us that they built all six of these sanctuary cities.
ASV
Deu 19:7 Wherefore I command thee, saying, Thou shalt set apart three cities for thee.
Deu 19:8 And if Jehovah thy God enlarge thy border, as he hath sworn unto thy fathers,
and give thee all the land which he promised to give unto thy fathers;
Deu 19:9 if thou shalt keep all this commandment to do it, which I command thee this day, to love Jehovah thy God, and to walk ever in his ways;
then shalt thou add three cities more for thee, besides these three:
Jos 21:43
So Jehovah gave unto Israel all the land which he sware to give unto their fathers; and they possessed it, and dwelt therein.
Jos 21:44 And Jehovah gave them rest round about, according to all that he sware unto their fathers: and there stood not a man of all their enemies before them; Jehovah delivered all their enemies into their hand.
Solomon possessed....
2Ch 9:22 So king Solomon exceeded all the kings of the earth in riches and wisdom.
2Ch 9:26 And he ruled over all the kings from the River [Euphrates] even unto the land of the Philistines, and to the border of Egypt.
2Ch 9:28 And they brought horses for Solomon out of Egypt, and out of all lands.
The Abrahamic covenant was fulfilled. Have you got another covenant or scripture to the contrary of these?