[mikesears]

2-34-3: A change of possession occurs when the opponent gains player possession during the down. The penalty says if there is no change of team possession

PENALTY says: 15 yards and a first down from end of last run when last run ends beyond the neutral zone and there has been no change of team possession, or otherwise 15 yards and first down from previous spot

Absent any verbiage in the rule that would support some "point in time" theory, we have to assume change of possession as occuring at some point during the down.

Therefore, if B gains possession DURING THE DOWN, penalty is enforced from the previous spot.

If A had not fumbled (a bad thing) and allowed B to get it (an even worse thing), they would have gotten the foul from the end of their run.