Returning the shabka after filing for khul'a
In Islamic law, khul'a is the annulment of marriage in return for compensation. According to the majority of scholars from among the salaf (first three generations of Muslims) and khalaf (their successors), khul'a is permissible in Islamic law. The legal evidence for its permissibility is the words of Allah the Almighty,
divorce is only permissible twice; after that, the parties should either hold together on equitable terms, or separate with kindness. It is not lawful for you (men) to take back any of your gifts (from your wives), except when both parties fear that they would be unable to keep the limits ordained by Allah. If you (judges) do indeed fear that they would be unable to keep the limits ordained by Allah, there is no blame on either of them if she give something for her freedom. [Al-Baqarah, 229]
Further evidence is the hadith narrated by Ibn Abbas (may Allah be pleased with them both) who said that the wife of Thabit Ibn Qays approached the Prophet and said, "O Messenger of Allah! I do not find fault with Thabit Ibn Qays in anything concerning his character or religion, but I do not want to disobey [the teachings of] Islam." The Prophet asked her, "Do you agree to return the orchard he gave you?" "Yes," she replied. The Prophet then told Thabit, "Accept the orchard and grant her a divorce" [Bukhari].
It is established that a custom which does not contravene Islamic law is one of the pillars of Islamic legislation. This is due to the non-Prophetic report from Ibn Mas'ud who said, "Whatever Muslims deem to be good is good in the eyes of Allah and whatever they them to be offensive is offensive in the eyes of Allah" [Ahmed]. The custom is that the shabka is part of the mahr (dowry).
If the couple agrees to separate through khul'a, or if the wife is granted khul'a by the court when they cannot reach an agreement on their own, she must return the mahr to the husband. This includes the shabka since it is considered part of the mahr.
Allah the Almighty knows best.