Bad after service from Avis Car Rental - Cape Town International Airport

My husband and I rented a car from the Avis branch at Cape Town International on Friday, 28th February 2020 until Monday, 2nd March 2020. The service on Friday was really good. The lady that assisted us was very friendly and professional. We enquired about the holding deposit and the waiting time before it gets refunded to a credit card. The agent advised that it takes up to 7 days, however; if we come into the office to speak to an agent, it is possible to speed up the refund. When we returned the car, we did do as told by the agent; however they said it is not possible and we had to wait the stipulated 7 days for our refund. Yesterday (5th March 2020), we received a receipt from Avis reflecting an additional amount of R200+ (fuel costs of 12l of fuel). This is NOT possible as we had refuelled the car before returning it. The agent even asked us if we had refuelled and we said yes. My husband even asked if the receipt from the garage was needed and the agent said no. My husband contacted Avis and all the agent could say was they will investigate. Furthermore, the customer service agent was quite rude to my husband. I do hope that this query will be given Urgent attention as we are really unhappy with the after-service received from Avis.