Payments Received with Paypal are not editable

Payments Received with Paypal are not editable

When we receive a payment through paypal, Zoho Invoice does not allow us to update that payment.
Since zoho does not automatically update currency conversion rates, this means that when we receive a paypal payment we have no way to enter the currency conversion rate applicable on that date. Zoho invoice automatically assumes it to be the same as on the invoice.
Is this not a bug? Has somebody else noticed this as well?

Thanks,
Prabhash