Agreement of sale extension

Dear All, We have entered into a sale agreement by paying an considerable amt of advance and has gone for housing loan. According to the agreement remaining amount will be paid within 45 days, i.e at the registration time.. This due date is falling on 23rd march.. But we are requesting the seller to extend it for one week i.e until 01 April.. Our only concern for this is to get applicable for new Gst rate of 5% instead of existing 8%. The seller is not ready for this one week extension.. More over sale deed document given by the seller to us is a wrong one and has incorrect information regarding the survey numbers of the property, which is recently observed by the banker while the loan is processing. we need to correct it and get it signed by the sellers in the meanwhile as to get the loan amount sanctioned.. Please let me know what is the procedure to be followed to extend the period for another 10 days. Thnx..