Contrary to numerous baseless accusations of theft that have been made against you here, the fault, if any, is with the company, and it is up to them to rectify it.
If you are now of the opinion that you are dipping into the card beyond what was due when you finished with them, it is open to you to request a statement from the provider, and if you get one, which you may not, as it may be the company's statement not yours, you will then be in a position to confirm or refute your opinion. Only then are you in possession of sufficient knowledge to be sure there is something wrong, and only then need you stop using the card if it turns out you are right. If you don't get the statement from the provider, you could request one from the company.
If you don't get either, you should probably just return the card at the point when you think you're done with it, but there is no reason for you to take a business risk on the timing of that of under-claiming your own money.
If the company (a) agrees with your assessment, (b) cares, and (c) does something about it in the form of a demand, you should then negotiate a settlement, which may not necessarily consist of the full amount, and need not consist of a single payment in full, but of instalments that suit you. Business is business, and there has been contributory negligence here. Unless the amount is into $US four figures it is fairly unlikely that either (b) or (c) will be true.
My father quit the Army in 1945 and received a letter demanding return of some overpayment or other. He wrote back asking for a proper accounting, and never heard another word about it. I did a similar thing when an employer demanded reimbursement of an alleged overpayment of annual leave, with the same result.
This is all accounting, and business, not a morality play. You should certainly forget all about crime, ethics, or moral obligations. It is a business stuff-up not of your making. Nobody is going to haul you into court over this.