Fake documents submitted to bank by builder

I am writing to seek your advice on a concerning issue that has arisen between myself and the builder of the flat I purchased 8 months ago. The flat has a loan from Renown Bank, and the builder is now claiming that the documents shown to me and submitted to the bank were executed by unauthorized employees. I would like your guidance on how to handle this situation, as I have already made payment to the builder's bank account, not to any individual employee's account. The builder is now accusing me of using fake documents and is requesting that I cover any potential losses that may have occurred. However, I believe that it was the bank's responsibility to verify the authenticity of the documents, as they collected all original documents directly from the builder's office. I do not feel that I should be held liable for any discrepancies that may have occurred. I would greatly appreciate your expertise in navigating this dispute and determining the best course of action to protect my interests. Thank you for your time and assistance.