Hi Acrylon,
What is the Maximum Payment amount on this invoice, as seen through Payment Control? I suspect it is 0.00.
If you partially pay an invoice, this Maximum Payment value will be set to 0.00, unless you have the Company Profile option to 'Set Amt Payables to Inv. Balance' enabled - from the System Options tab. You likely need to edit the Max Payment to the Invoice balance, and probably want to enable this company profile option.
The Remaining Amount takes into consideration the Max Payment value.
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Regards,
Softrak Tech Support