To fix:

Edit the vendor
On the Transaction tab double click on the invoice
Change the Maximum Payment Amount to equal the Current Amount

You can now pay the balance owed.

To prevent this from happening again:

Edit the Company Profile
ON the System Options Tab - turn on "Set Amt Payable to Inv. Balance"
_________________________
Douglas Dickie
AccSys Solutions Inc
Phone: 1.888.534.4344
ddickie@accsyssolutions.com