Well, I would suggest starting by not trying to subtract a name from a number - it generally doesn't work too well...
I would also suggest using int.TryParse to convert the textbox values to numbers - with error reporting if the user types a non-numeric value.
Dim charge As Integer
If Not Integer.TryParse(txtCharge.Text, charge) Then
...
Return
End If
Dim payment As Integer
If Not Integer.TryParse(txtPayment.Text, payment) Then
...
Return
End If
Dim balance As Integer = charge - payment
txtBalance.Text = balance.ToString()