ASP.Net GridView RowUpdating Error converting data type varchar to numeric

Hi to all,

Error converting data type varchar to numeric.

I get an exception from my application. It is thrown on cmd.ExecuteNonQuery(). I tried to update a row in database. In db the type is decimal(10,2), I convert it to decimal when it updated but does not work. How to resolve this convertion?

    protected void GridViewIncomes_RowUpdating(object sender, GridViewUpdateEventArgs e)
    {
        SqlConnection sqlCon = new SqlConnection(CS);
        TextBox tbDenumire = (TextBox)GridViewIncomes.Rows[e.RowIndex].FindControl("tbDenumire");
        TextBox tbSuma = (TextBox)GridViewIncomes.Rows[e.RowIndex].FindControl("tbSuma");
        TextBox tbData = (TextBox)GridViewIncomes.Rows[e.RowIndex].FindControl("tbData");
        
        TextBox tbDescriere = (TextBox)GridViewIncomes.Rows[e.RowIndex].FindControl("tbDescriere");
        int VenitId = Convert.ToInt32(GridViewIncomes.DataKeys[e.RowIndex].Values[0]);
        string sql = "UPDATE Venituri SET Denumire='" + tbDenumire.Text + "',Data='" + tbData.Text + "',Descriere='" + tbDescriere.Text + "',Suma='" + Convert.ToDecimal(tbSuma.Text) + "' WHERE VenitId=" + VenitId + "";

        SqlCommand cmd = new SqlCommand(sql, sqlCon);
        sqlCon.Open();
        * cmd.ExecuteNonQuery();
        sqlCon.Close();
        if (cmd.ExecuteNonQuery() == 1)
        {

            lblSuccessMessage.Text = "Actualizat cu succes!";
            GridViewIncomes.EditIndex = -1;
            GridViewIncomes.DataBind();
        }
       
        lblSuccessMessage.Text = "";
    }

<asp:TemplateField HeaderText="Suma">
    <EditItemTemplate>
        <asp:TextBox ID="tbSuma" runat="server" Text='<%# Bind("Suma") %>'></asp:TextBox>
    </EditItemTemplate>
    <ItemTemplate>
        <asp:Label ID="lbSuma" runat="server" Text='<%# Bind("Suma") %>'></asp:Label>
    </ItemTemplate>
</asp:TemplateField>

This looks the database:

[VenitId]   INT             IDENTITY (1, 1) NOT NULL,
[Denumire]  NVARCHAR (20)   NOT NULL,
[Data]      NVARCHAR (50)   NOT NULL,
[Descriere] NVARCHAR (MAX)  NULL,
[Suma]      DECIMAL (10, 2) NULL,