Do I have to pay back full amount of promised bonus when only one fourth has been paid to me?

I signed an agreement to receive a bonus of 7500 for a nursing job with it to be paid in four installments in eighteen months - I received the first in my most recent paycheck which I think is sketchy due to not knowing how much of the 1000 dollars taken out for taxes was for my pay or the bonus - either way, I do not want the bonus because I am unsure if I want to stay due to finding out some issues with the company quickly in the past six weeks of starting. I read the agreement again, and immediately notified the company for clarification and letting them know that I do not want the money. The way it reads, even though I have only received one fourth of the money, I would have to pay back the entire 7500. That does not seem legal to me? Please advise.