Builder is asking me to pay the CAM maintenances charges for the period when I do not own the flat.

Hi, I purchased a flat from first purchaser under assignment agreement in Dec-20 in Bangalore. The society is from a popular builder. The first purchaser has taken possession from Nov-19. As per assignment process the first owner has to clear all the pending charges including maintenance charges with CRM. We confirmed and the CRM told that their are no pending charges (they didn't gave us in written). However on Mar-21 CRM bills me for the maintenance charges from Apr-20. When asked they say they have already informed the first purchaser that the billing of CAM will start from Apr-20 irrespective of date of possession and I have to pay those charges as the flat is in my name now. And their is a delay in billing so their were no pending charges during assignment process. When I told them to collect from first owner as I was not the owner of the flat for the period they have billed me. They say that's how I see it but I have to pay those charges else I will be in trouble. The first owner says that he asked them and they told everything was clear and hence he is not going to pay any charges. What should I do or what should be my next step? Thank you for your time.