Posting payment (FP05) resulted to Balance in Document currency is not Zero

A customer with a payment plan has a balance of $250 as March 2009 invoice.

The customer paid $400 after getting the invoice. In posting the payment via FP05, the error message came out.

1) What is this error message?

2) How come the difference of $150 did not automatically go to Payment on Account?

3) Is there a way to automatically do that instead of manually performing post processing steps in FP05?

Balance in Document currency is not zero, posting not possible

Message no. >0003

Diagnosis

You are attempting to post a document for which the balance is not zero in one or more currency.

System Response

During online processing:

Error message requiring a correction.

During background processing:

Error message causing processing to be terminated.

Procedure

When working online:

Correct the line items entered. If the amounts entered balance to zero, and this error message still appears, there is a program error. In this case, treat the error as if it occurred during a background processing.

Background processing

Contact your system administrator.