Did Abraham and Isaac actually possess the promised land in their lifetime?

Abraham and Isaac did not actually possess the promised land in their lifetimes. They continued dwelling among the Canaanite tribes and did not own the whole land. The promise remained future: God swore to give their seed all these countries, but that fulfillment had not yet come during their days.

Les explains that Abraham and Isaac are still dwelling amongst all the Canaanite tribes that were there when Abraham came in. They haven't got the whole land, it isn't theirs, and they are still dwelling in the midst of the Canaanites. But the promise is that the day is coming when they won't have to put up with the Canaanites, and God is going to give them the land. Les then reads verse 3: "I will give thee all these countries, and I will perform the oath which I swear unto Abraham." So the Abrahamic Covenant comes in here, promising future possession, not immediate possession during their lifetimes.

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.