Allow Numbers Only In UserForm TextBox But No Negatives

I have the following code in a user form, attached to a data field that the user should fill. This code should alert the user if he enters a negative number i.s.o. a positive (and vise versa). Due language related issues, it is quite possible that the user will enter (by mistake) a negative sign but it will be at the end (500-) and the user will not notice this mistake. For some reason, my code does not pick up on this, and does not pop up the error message. Therefore, I decided that I need to check if the value entered by the user also contains a “-“ at the end of the string. This, I believe, will take care of the problem.

In excel, I would simply enter a formula with Mid and Len (to check if the last character is “-“) , but I don’t know how to implement it in my code.

Sub txt_sum_BeforeUpdate(ByVal Cancel As MSForms.ReturnBoolean)
'Verify that a negative sign was added for expenses
With Me.txt_sum
Select Case cbo_act
Case "Expense",
If Me.txt_sum.Value > 0 Then
MsgBox "Number must be negative"
End If
Case "Income"
If Me.txt_sum.Value < 0 Then
MsgBox "Number must be positive"
End If
End Select
End With
End Sub