What should a person do if they are denied their financial rights and entitlements, and is it permissible for them to take a portion of their usurped right through "al-Zafar" (gaining the upper hand/taking by force)?

It is not permissible to betray a trust. However, you have the right to address the official to inform him of the failure to implement his decision, so that he may lift the injustice from you. It is also possible that the direct official has the right to object to decisions made by those above him.

You must return what you took previously, even if by stratagem, based on the Prophet's (peace be upon him) saying: "What the hand has taken, it is responsible for until it returns it."