Bava Basra 100 - 106
Walking The Land, Paving The Way
Avraham Avinu was commanded by Hashem to walk the length and breadth of Eretz Yisrael which was being presented to him as a gift. The majority view of the Sages rejects the opinion of Rabbi Eliezer that this is proof that one who acquires a field can finalize the transaction by simply walking upon it. Their view is that one can only render a transaction irrevocable by demonstrating his ownership through making some improvement in the land. Avraham Avinu's walking was for a different purpose - to pave the way for his descendants to easily conquer the land.
This is best understood by applying our sages' principle of "Whatever happened to the forefathers happened to their descendants" which Ramban elaborates upon in his commentary on Chumash. The walking of Avraham is symbolic sovereignty which transforms in the future to concrete conquest by Joshua.
- Bava Basra 100a