bad landlord

My former landlord wants to charge me several hundred dollars for the stains on the new carpet. I went to the apartment and found the carpet almost had been cleaned completely. He didn’t change the carpet and the new tenant has already moved in. I think it’s unfair. He occupied my deposit and he wants to take more money. My friends suggest me ”just ignore it”. But I’m afraid the landlord will damage my credit. I don’t know how to deal with it. Please help me. Also, I want to know how he would do if he wants to damage the credit. Thank you.