Mistake on account number leading to loss of cash?

A few days back we deposited cash into an absa account and only realised after some days that we entered the last digit on the account number wrong, we inquired with absa if the account number we mistakenly deposited to exists and one of the call centre people said the account doesn't exist so we can go to the bank and they should help reverse the cash. We went to the bank only to be told they can't help us without a slip and they can't tap into someone's account without checking; we understood that a slip is valid proof for the deposit but the slip is lost and we have all the details of the deposit made like date,reference,atm used, we can even cross check the account numbers to show the simple mistake made as we make that deposit every single month for the past 6 or more months, the incorrect account belongs to No one so it doesn't make sense why the bank is not willing to even cross check the details and find other proof as this is a huge loss to us. Are they trying to keep the money without any bother to investigate? We will highly appreciate it if someone could understand and help us get the cash back