Parcel delivery got late so party not accepting bill in that month

i have sold goods in august month and invoice was made on 30/08/2018

now in transit the parcel got missed due to transporter's carelessness and the party received the parcel in october month that is 03/10/2018

now party is saying that they have already done all the purchase entries and other formalities of august month and demanding my sale bill generated in august month to be CHANGED AND make it in OCTOBER month.

now please clear my query that how should i proceed in this case so that none of us gets trouble later on due to accounts mismatch