Chasidut к Берешит 17:19

Once we appreciate this we can also understand why in ‎‎parshat Lech lecha prior to Avram being commanded to ‎circumcise himself, G’d promised him that he would father a son ‎from Sarah. (Genesis 17,15-16) This meant that the reward for all ‎the commandments Avraham would perform subsequently would ‎not be offset against accumulated merits in respect of miracles ‎G’d would perform for him. These “miracles” would be viewed as ‎fulfillment of what G’d “owed” him, i.e. promises made but not ‎yet fulfilled.‎