What does Hashem mean when He now tells Avraham that He is giving the land to him and to his children?
Ramban: He was now bequeathing the land to Avraham, who would acquire it immediately on behalf of his children. 1
Seforno: He meant that, although it was only Avraham's children who would inherit the land, Avraham himself would enjoy great prestige among the inhabitants of the land, who would view him as a Prince of G-d.
Ramban: As the Gemara states in Bava Basram 119b 'It is an inheritance for Yisrael