There is a user exit which recalculate due date if the Payment Term used in the document is included in Table ZFI_PAYTERM, This calculation causes the field Days 1 to have a negative value.
Since it is not advisable to directly update SAP standard Tables to remove the negative value and the document has been reversed, Can we tag this incident as an isolated case and ensure that in the future that new Payment Terms to be added in Table ZFI_PAYTERM will be reviewed carefully.