You'll need some code for that. Assuming that the sum owed is in A1 and that you are entering payments in column C try this. Right click the sheet tab, select View Code and paste in
Private Sub Worksheet_Change(ByVal Target As Range) If Target.Column = 3 Then Application.EnableEvents = False Range("A1").Value = Range("A1").Value - Target.Value Application.EnableEvents = True End If End Sub
Then press ALT + Q to close the code window.