Merit and Inherit
In his commentary on this passage from this week's Torah portion, Rashi cites the following explanation of the Midrash for the stress on "He will have given to you".
It shall be in your eyes as if you received the land the day you enter it, is the Divine message, and not merely an inheritance from your forefathers.
There is a special feeling when receiving a precious gift one has earned rather than automatically inherited. The mitzvah which follows the above-mentioned introduction – the sanctification of the first-born – served as a merit for the Jewish nation to acquire their land. It was therefore communicated to them that they should view their entry into the land as a gift they acquired with this and other mitzvot rather than just an inheritance.
This relationship between merit and inherit continues throughout history. Only by increasing our merits by fulfilling our spiritual potential can we hope to inherit Israel forever.