With the standard invoice templates supplied with SliQ Invoicing the total field shows the total amount on the invoice minus any payments made, i.e. the total field shows the total amount outstanding by the customer.
If you would like to change this so that the total always shows the original, unpaid invoice total and does not subtract any payments, you will need to edit the invoice template and change the field mapping of the Total field to be TotalExPayments.
Editing the invoice template is an easy step - go to the main Setup tab, then select the Template Setup subtab. Then press the Edit button next to the invoice template dropdown. When the template editor opens, click on the Total field and then in the field properties shown on the right of the editor, select TotalExPayments from the dropdown.