Subtraction in C# using SQL Server stored procedure

Question

I have to create a banking app for my class using c#. My problem is that when I try to subtract an amount of money in one account, let's say 10$ in one account, the value in my table goes up by like 1000$ instead of going down of 10$.

first of all i have a stored procedure name 'retrait' that goes like that

CREATE PROCEDURE retrait
    @montantRetrait money,
    @numCompte int
AS
BEGIN
    UPDATE tblCompte
    SET solde = (SELECT SUM(solde) FROM tblCompte) - (SELECT SUM(@montantRetrait))
    WHERE numeroCompte = @numCompte
END

Mnd my C# code is like this:

private void btnConfirmer_Click(object sender, RoutedEventArgs e)
{
    int typeTransaction = cmbTransaction.SelectedIndex;

    if(verifierSaaisie())
    {
        switch (typeTransaction)
        {
            case 0:
                // Creation de la commande avec procedure stocker
                commande = new SqlCommand("retrait", connexion);
                commande.CommandType = CommandType.StoredProcedure;

                // Ajout de valeur a nos parametre
                commande.Parameters.Add("@montantRetrait", SqlDbType.Int).Value = textMontant.Text;
                commande.Parameters.Add("@numCompte", SqlDbType.Int).Value = cmbProvenance.Text;

                try
                {
                    // ouverture de la connexion
                    connexion.Open();

                    // Execution de la commande
                    int ligne = commande.ExecuteNonQuery();

                    if (ligne != 0)
                    {
                        if (MessageBox.Show("Transaction effectuer avec succes. Voulez vous effectuer une autre transaction ?", "Message", MessageBoxButton.YesNo, MessageBoxImage.Question)== MessageBoxResult.No)
                        {
                            this.Close();
                        }
                    }
                }
                catch (Exception ex)
                {
                    MessageBox.Show(ex.Message);
                }
                finally
                {
                    connexion.Close();
                    cmbTransaction.SelectedIndex = -1;
                    cmbProvenance.SelectedIndex = -1;
                    cmbDestination.IsEnabled = true;
                    textMontant.Text = string.Empty;
                }
                break;
            }
        }
    }
}

Any advice would be great.