I received a sign-on bonus upfront of $1500 without any stipulations in the employee handbook or in my acceptance letter. After 78 days I resigned and gave my 4 weeks notice. They asked me to leave immediately. Now, 6 weeks after my last day on the job I have received notice that I need to repay the $1500 asap. I am wondering what my legal obligations are since I was not notified of any conditions when I received the money?

There are always stipulations on "sign on bonus", it would have been made during your hiring negotiation.