Call in Variable File Directory and Filename from Cell into Save As Prompt

Try this:

Option Explicit

Private Sub Workbook_BeforeSave(ByVal SaveAsUI As Boolean, Cancel As Boolean)
Dim fFullPathName As String, fTEST As String

Application.EnableEvents = False
fFullPathName = Sheets("Input Sheet").Range("B1").Value

'Test if file already exists
If Len(Dir(fFullPathName)) > 0 Then
    If MsgBox("The file '" & fFullPathName & "' already exists. Do you wish to overwrite it?", vbExclamation + vbYesNo) = vbYes Then
        Application.DisplayAlerts = False
        Cancel = True
        ThisWorkbook.Save
        Application.EnableEvents = True
        Exit Sub
    End If

End If

fTEST = SaveItAs(fFullPathName)
Application.DisplayAlerts = False
If fTEST <> "False" And fTEST > "" Then ThisWorkbook.SaveAs fTEST
Cancel = True
Application.EnableEvents = True
End Sub

Function SaveItAs(MyFile As String) As String
On Error GoTo ErrorExit

    With Application.FileDialog(msoFileDialogSaveAs)
        .AllowMultiSelect = False
        .ButtonName = "&Save As"
        .InitialFileName = MyFile
        .Title = "File Save As"
        '.Execute
        .Show
        SaveItAs = .SelectedItems(1)
    End With

ErrorExit:
End Function