Consumer Redressal

Dear Sir/ Madam, I have a query to be asked. I have purchased a flat and it was mentioned in the contract that I will pay the property tax " from the date of purchase". However, I got the first property tax demand receipt from the department and it is mentioned that i will have to pay the past dues also which are before the date of my purchase. I think the builder has done bribed the tax authorities and transfered the past tax due to my name even though it is clearly mentioned in the contract that I will be liable to pay the tax only after the date of my purchase. Please advice what shall I do. Awaiting your valuable response Imran