Assuming I read your request properly, the code below may help your efforts.  
It overrides the built-in Save function.  One bug in the code below is that
it crashes if the Save As dialog comes up and the user hits Cancel.  But
that's what error-handling is for.

Sub FileSave()
   
   'The following line defines the string expected in the footer
   Const REQUIRED_STR = "Put defined string here"
   
   With ActiveDocument.Sections(1).Footers(wdHeaderFooterPrimary).Range
   
       'Check footer text (first section main footer only)
       If InStr(1, .Text, REQUIRED_STR) = 0 Then
       
           'Prompt user for footer text
           Dim newStr As String
           newStr = InputBox("Footer text should include '" & REQUIRED_STR
& "'.  Enter new footer text:", "Enter string")
           
           If newStr = "" Then 'User didn't input any text, so abort save
procedure
               Exit Sub
           Else 'Place specified text into footer
               .Text = newStr
           End If
       End If
       
   End With

   'Save file
   ActiveDocument.Save

End Sub