How to create VB.NET subtract from purchase table using sales table quantity

Item     qty
1        100kg
2        200kg

item      qty
1        20kg
2        30kg

Strcmd = " UPDATE stock SET qty = stock.qty - sales.qty FROM stock INNER JOIN sales ON stock.item = sales.item"
            'Strcmd = " insert into sales (item,qty)values (@item,@qty)"
            conn = New SqlConnection(Strconn)
            cmd = New SqlCommand(Strcmd, conn)
            cmd.Parameters.Add("@item", SqlDbType.VarChar).Value = TextBox1.Text
            cmd.Parameters.Add("@qty", SqlDbType.Int).Value = TextBox2.Text
            conn.Open()
            dr = cmd.ExecuteReader()