A.T. Robertson Commentary Galatians 3:18

"For if the inheritance is of the law, it is no more of promise: but God hath granted it to Abraham by promise." — Galatians 3:18 (ASV)

The inheritance (η κληρονομια). Old word from κληρονομος, heir (κλερος, lot, νεμομα, to distribute). See on Mt 21:38; Acts 7:5. This came to Israel by the promise to Abraham, not by the Mosaic law. So with us, Paul argues.

Hath granted (κεχαριστα). Perfect middle indicative of χαριζομα. It still holds good after the law came.