All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
What you did is Islamicly prohibited since you were working, and thus you were not eligible for Social Welfare Benefits which were conditional to being unemployed. Taking this money was based on lying and it was also cheating. The fact that your salary was not sufficient for you does not allow you to lie and cheat. It was possible for you to stop their aid and depend on your work only. The Prophet said: "One's soul will not die until it would have consumed its (designated) provision, so fear Allaah and seek lawful provision. However, if your provision is delayed, do not seek it through disobeying Allaah." [At-Tabraani] Dear questioner, you have to repent to Allaah and give back the money that you took from the government while you were working, as it was money that had been taken illegally, and it is incumbent upon you to return it to its owners.
Allaah knows best.