payment not able to save it.it raising exception occured when performing payment creation

Hi,i try to create usd payment.but not able save entry.it raising exception occured when performing payment creation.
i try to match two invoices.
one invoice standard invoice.
second one credit memo invoice.
but while saving entry it giving like erros
exception occured when performing payment creation.
what reason it given that erros.
pl replay me.
reg
sasi.