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
Dim payment As Integer
If Not Integer.TryParse(txtPayment.Text, payment) Then
Dim balance As Integer = charge - payment
txtBalance.Text = balance.ToString()