‘For if the inheritance is of the Law, it is no more of promise. But God has granted it to Abraham by promise.'
The later inheritance, to be obtained by fulfilment of the Law, was dependent on fulfilling the Law. If that fulfilment failed the promise failed. But the promise to Abraham was given a long time before the Law ever existed, and at the time when it was given, it was not dependent on anything but the faithfulness of God. It was a free unfettered promise, and it included the nations of the world. Thus it has nothing to do with the Law, and can be enjoyed without recourse to the Law. It preceded the Law and transcends the Law.