How to abort a siege assault without breaking the siege?

It is bit different: As attacker, you cannot sally (because sally is defender trying to break siege). But that is only words.

As attacker, you can only try to mount final assault (and if you fail, you end siege)

On the other hand, if defender sally and don't win, siege continues (but he does not replenish losses in his normal nor garrison units). I don't know whether he takes aditional loses aprat from suffered in actual battle.

In M2TW it was similar, but different, bacause there the sally was actualy from besieged castle/town and defender needed to return inside, and attacker (besieger) could win siege even if fight was initiated by besieged. In S2 besieger wins siege in sally battle only if he wipes out defender.