God in Gen 15 made a covenant agreement (abraham was put to sleep. so in effect, God made a one way “I will” covenant, or what many call an unconditional covenant meaning the one party said “I will do this”, yet did not make Abraham or his party agree to do anything, it is not binding on abraham or his party to keep their side of the agreement, because none was given, ie it is unconditional (as apposed to the mosaic covenant which was conditional) or what we consider a 2 party covenant, where one party agrees to do this, and return the second party agrees to do that, if one party failed to live up to their obligation, the covenant is broken, and deemed cancelled. The other party is thus no longer required to do what it promised,
this agreement, was to give them land, it was an unconditional covenant,