Wrongly Invoiced in Credit Memo

Hello Gurus,

we are creating an Credit Memo Using "MIRO".

we wrongly debited to the customer and credited to GRIR Account.

Posting Keys Used(21-for customerr and 96- for GRIR account.

Actual Payment to be sent is "$8700.00"

By doing this transaction which we wrongly debited the customer is $625.

And we had sent a check for $8075 which he already cashed it.

Now i need to know how do i reverse the transaction of wrong debit and credit and send a fresh payment for $625.

when i am trying to do manual adjustments to the GRIR account i am getting error "can only be posted to internally in code"

please advise me in this regard.

thanks,

anand