If you want to know what legal services the $500 unpaid bill is for, you can ask: "Please let me know what services the $500 is for. Please send me a detailed invoice." This question would make sense if you agreed to pay your lawyer by the hour. The lawyer should give you a bill every month showing how much time he/she spent on your case, and what exactly he/she was doing.

Your sentence may be modified to: My lawyer told me we note that you have not paid the amounts of 500$ for your case. What are you trying to ask about?

Are you trying to say that you knew nothing about an unpaid payment. If that is the case than you can say something like: I was unaware that a payment was missed. My apologies, please send another bill for this payment. Thank you. Hope this is helpful.

I think John gave a good answer. I just want to emphasize one thing that you will use a lot. In English we write $500 (as he did), and not 500$ when we use the symbol $, although we pronounce it and write it in full "500 dollars". I know you will talk about money in the future.