Did Abraham and Isaac already possess the promised land when God gave the promise?
No. At the time of the promise, Abraham and Isaac were still dwelling among the Canaanite tribes and had not received the whole land. The land was not yet theirs; they remained in the midst of the Canaanites. God promised that the day was coming when He would give them the land.
Les explains that Abraham and Isaac were still dwelling among all the Canaanite tribes that were there when Abraham came in. They had not obtained the whole land, it was not yet theirs, and they remained in the midst of the Canaanites. The promise, however, was that the day was coming when they would no longer have to put up with the Canaanites, because God was going to give them the land. In verse 3, God says, "I will give thee all these countries, and I will perform the oath which I swear unto Abraham, and I will make thy seed to multiply as the stars of heaven, and will give unto thy seed all these countries, and in thee shall all the nations of the earth be blessed." Les also notes that the Abrahamic Covenant comes in here, and that in Abraham's seed all nations of the earth would be blessed because Abraham obeyed God's voice and kept His charge, commandments, statutes, and laws. This shows that the possession of the land was still future when the promise was given.