How to prove that sender tempered with his own email content


New Email
Hi everyone, am new to a discussion forums, so be patient with me please. My acquaintance has emailed to me his bank details so that I could transfer money to his account. I have transfered the money as per the email, printed and scanned the bank transfer receipt, and used a reply button with attaching the bank transfer confirmation and wrote confirmation of my actions. The money has been debited from my account by my bank. However, later I have received another e-mail from my acquaintance, which looks like continuation of our e-mails, where he claimes, that I have missed a digit in the bank account number and that it is my fault. Surely enough, in the part of his e-mail where was the bank account one more digit was added now. I know, that he tempered with the content of his own email formerly sent to me. I would be plainly stupid attempting to credit someone's else account...
Please, how can I get an evidence to prove to him, that he changed/edited his own email content?
I don't want to use a reply button to his last email knowing that he edited the bank account number in lower parts of the email...
What are your thoughts guys? Regards Dale