Sir
According to rule 96 A(1)(b) of CGST Rules,2017 exporter has to pay the tax due along with the interest specified under sub-section (1) of section 50 within a period of fifteen days after the expiry of one year, or such further period as may be allowed by the Commissioner, from the date of issue of the invoice for export, if the payment of such services is not received by the exporter in convertible foreign exchange or in Indian rupees, wherever permitted by the Reserve Bank of India.
Therefore in your case since the payment is not received within one year from the date of export invoice you have to pay IGST at appropriate rate along with interest.