If your business is set up for GST then when creating an invoice in the tax type column you can select “NON-GST”
Okke at this stage is not optimised to handle currency conversions. I would recommend consulting an accountant regarding how to enter this one in. However, what I can say is If you record the payment as an over payment against the invoice it will then create a credit for your customer. Or alternatively you can edit the invoice to the amount received. This is completely up to you and what your accountant would recommend.
If you have not recorded the invoice payment in cashflow then you can just go through the invoice list and find the invoice you would like to edit.