Problems with calculations and conversions in Visual Basic Form

Answer 1

You need to explicitly cast your variables when pushing them into the text box.

If TextBox3.Text < 1000 Then
    TextBox4.Text = ""
End If
TotalSalesAmt += TextBox3.Text
TotalComAmt += TextBox4.Text
GrandTotal += TextBox2.Text

For instance your TextBox3.Text above is a String, not a Decimal. Your trying to move through types without casting them to the appropriate type.

Answer 2

TextBox2.Text = Decimal.Parse(CommissionCalc(SalesDecimal))

You are using Decimal.Parse incorrectly. Decimal.Parse is used to parse a String to a Decimal type, you seem to be using it to do both a string to decimal and decimal to string. If you want to get the string representation of the decimal value you should use ToString. In the example above form your code, CommisionCalc returns a Decimal number, you need to call CommissionCalc(SalesDecimal).ToString() not Decimal.Parse()