For if the inheritance [be] of the law, [it is] no more of promise: but God gave [it] to Abraham by promise.
Note 24 at Ga 3:18: Receiving by Law and receiving by promise are opposites. Law involves effort (works). Promise provides as a gift (grace). We are either saved by works or by grace, but not a combination of the two (see note 2 at Ro 11:6).