Who is requesting the $500.00? Is it the store? If so, I usually tell my clients not to worry about paying the store any money. Yes, they could sue you in civil court, but it is unlikely.
If you have already plead guilty or no contest to the criminal charges and are on probation (which it sounds like from your post) this is a civil collection matter. Many times having an attorney write Target a letter explaining that there has been no loss, that their restocking fee is too high or that their damages are not justified will suffice. They might sue you they might not but it is worth at least trying to negotiate the amount down. if they sue you you will have to respond or risk a default judgement. It sounds like they have no damages to speak of or at least not that justify the $500. See if the attorney that helped you get probation will help to stop this collection.
Although my intent in answering this question is to aid you in the legal process, my answer does not establish an attorney-client relationship in any way. You should seek the advice and counsel of a qualified attorney in your community to evaluate your legal needs and to advise you. No Attorney-Client Relationship is created without the specific intent of both parties.
It sounds like you have received a demand letter that is generated after someone is caught shoplifting. The law provides that businesses can recoup the expenses they incur in trying to stop theft by assessing fees to those they catch. The demand letter is sent to every person who is caught shoplifting.
If you do not pay the demand, then the only way that they have to make you pay is to sue you. Most lawyers tell clients that they do not need to bother paying the amount because of the great cost incurred by companies to sue a person - they must hire a lawyer (beyond the lawyer who sends the form demand letter), pay a filing fee, work on the case, etc. While the could win such a lawsuit, it is very likely that the person who was caught stealing is essentially judgment proof - does not have the money from which the company can recover. So, it is throwing good money after bad and companies just do not do it.
Worry about your probation and follow the rules of probation exactly. Do not worry about the demand letter.
Although I have answered the question to try to help you, you should consult with a lawyer in your area in person on the matter. In addition, my answer does not establish an attorney-client relationship between us.