Gst invoice for the year 2017-2018 can be ganarated in current period.

Dear Sir, in the financial year 2017-2018 we raised a proforma invoice of rs 2000/-, party made the payment  along with other previous tax invoice. the issue is by mistake we did not generate a tax invoice for Rs 2000/-. we have filed the returns and also finalize our accounts. Now the  party is asking for tax invoice. Please suggest me in that case what has to be done, can i raise a credit note or raise tax invocie in current date. pls suggest .thank you