Writing off an underpayment

Does anyone know how to write off an underpayment, an overseas client has paid me and somewhere in the currency conversion it is £2.00 less than the invoice amount, whats the best way to write this off as im not going to ask them to pay the missing funds?

Thanks

You could just apply a credit note to the difference:

Thank you, i did think that might be the way but i wasnt sure if i was missing something