Urgent help please

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 one document given by the seller has incorrect information regarding the survey numbers.. we need to correct it meanwhile.. Please let me know what is the procedure to be followed to extend the period for another 10 days. Thnx..