Rental Office Wants More Money For Their Mistake!?

My rental office discounted my rent this month by $500. I paid my rent, less the $500 discount, only to be later told, after they accepted my discounted rent payment, that the discount was intended for another tenant and not me. I have now been told that I have to pay the $500 I was originally discounted or be evicted. I have a rent bill which indicates the discount in my name and for my unit. I paid rent with no problem and was issued a receipt. If this was a mistake on the part of the rental office then they had ample time and opportunity to catch their mistake. After they accept my payment can they then force me to pay more because they made a mistake on my bill? I usually pay $1300 for rent, but was asked in writing to only pay $800 because of a $500 credit for the month of June. I now know that the discount was not for me and had the rental office caught their mistake before I paid the discounted amount I would have complyed, but to demand I pay more after they have already accepted my rent payment seems a little unethical. Is this legal and do I really need to pay them? Even if the discount was not for me, my rent payment should not have been accepted if that was the case. I feel that by accepting my payment they basically accept any liability that comes along with it. For example, mistakingly giving me someone else's discount. Am I responsible for paying back the discount they accidently gave me even though they have already accepted my rental check for the discounted amount? Thank you for your time and help. By the way I live in Virginia.