Paul's point was that God's promise was to Abraham "and to your Seed", whom Paul said was Christ. The “inheritance” is what was promised. And that “inheritance” is the permanent possession of the Land. According to Paul, the Law, which provided a temporary and conditional possession of the land, cannot overturn the unconditional promise of the inheritance given to Abraham. Since Christ is the ‘Seed’ to whom the inheritance was promised, and since Christ had not yet come when Joshua led the Israelites into the Land under the Mosaic Covenant, it is obvious that the Abrahamic Covenant was NOT fulfilled in Joshua's day. It could not be fulfilled without Christ.