Gst amount mismatch

we issued a invoice with 18% gst to client in January 2021.
we show invoice in gstr1 with 18%.
client made payment basic amount with gst 5% in April 2021.
he says that he take itc of 5%due to mistake.
our 13% gst amount on hold .
we reminding customer to pay balance amount.
now they reconcile it.
now September returns filed.
how can client claim itc of difference amount & make payment to us.
what can we do?