Because the foul occurred during the loose ball portion of the play "the run which precedes a legal forward pass" (10.3.1.note) this would be enforced from the previous spot. A may decline the FM penalty and take the results of the play.
The "tack on" enforcement does not come into play in this case. The exception only applies to a passer who has been roughed and the pass is completed beyond the LOS.
In the OP situation the QB does not become a passer until he passes the ball and therefore does not qualify for being "roughed" while he is scrambling around. Had the QB been roughed, then the penalty can be added to the end of the run on a completed pass beyond the LOS (10.5.5), Otherwise is it simply a loose ball/previous spot basic spot.
The other caveat to this is any PF acts against the passer (which would include a FM, etc) even if they are not late put committed in the act of passing can qualify for roughing if it is against the passer. Yet, these PF do not qualify as roughing and therefore are not "tacked on" unless and until they are committed against a passer.
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