What is the Sharia ruling regarding the Zakat money that I was given, knowing that I did not need it due to the existence of other financial assistance that sufficed me, and I am still keeping it?

If you were eligible for zakat (definition) at the time of receiving it due to your poverty or destitution, then you have acquired ownership of it and are not obligated to return any remaining portion. The poor and the needy acquire ownership of zakat upon taking possession of it, and they are not asked to return it if they become self-sufficient thereafter. However, if you were not eligible for zakat, then you do not own what you took, and you must return all of it. And if the zakat is destroyed while in the possession of someone who received it but was not eligible for it, then he is liable for it.