What is the evidence from the Qur’an and Sunnah that the "shabaka" (engagement gift) should be returned to the suitor if the engagement is broken off by the fiancée, considering it a part of the mahr (dower)?

The engagement "shabaka" (jewelry) is either a gift or part of the dowry. If it is a gift, it belongs to the fiancée, and the suitor has no right to reclaim it. However, if it is part of the dowry, and the engagement is then broken, it rightfully belongs to the suitor, and it must be returned to him unless he waives his right to it.