Both require knowing what cell(s) you've used the =TextNumber(...) formula in.
#1-- store the value of those cells in variables before copying the sheet to the new workbook then write those values to the same cells in the new workbook before saving. Try something along the lines of this in your original macro
Code:
Sub SaveInvWithNewName()
Dim NewFN
Dim variable1
With ActiveSheet
variable1 = .Range("G40").Value
.Copy
End With
'the newly created sheet automatically becomes the active sheet
With ActiveSheet
.Range("G40").Value = variable1
End With
'
' rest of your code here
'
End Sub
#2-- much more complex