Using executeScalar() in place of executeReader()

I am learning webapi and working on a small project.(i know this question is not related to webapi, but just to get to the scene).So i have this function called get_balance(string accountNumber) which returns the balance.

-- Function: deposit.get_balance(character varying)

-- DROP FUNCTION deposit.get_balance(character varying);

CREATE OR REPLACE FUNCTION deposit.get_balance(deposit_account_number_
character varying)
RETURNS money AS
$BODY$
DECLARE _account_number_id bigint;
BEGIN

SELECT account_number_id INTO _account_number_id FROM
deposit.account_holders WHERE account_number = $1;
RETURN(
SELECT
SUM(COALESCE(credit,'0')) - SUM(COALESCE(debit, '0'))
FROM deposit.transaction_view
WHERE account_number_id = _account_number_id
);
END$BODY$
LANGUAGE plpgsql VOLATILE
COST 100;
ALTER FUNCTION deposit.get_balance(character varying)
OWNER TO postgres;

Since this is a Webapi project i have the controller as AccountsController

 [HttpPost]
    [ActionName("balance")]
    public IHttpActionResult GetBalance([FromBody]AccountBalance account)
    {
        account =
    BusinessLayer.Api.AccountHolderApi.GetBalance(account.AccountNumber);
        return Ok(account);
    }

The business Layer

public static AccountBalance GetBalance(string AccountNumber)
{
    return DatabaseLayer.Api.AccountHolderApi.GetBalance(AccountNumber);
}

And the Db Layer

public static AccountBalance GetBalance(string AccountNumber)
{
    AccountBalance result = null;
    using (var conn = new NpgsqlConnection("Server=localhost;UserId = postgres; " + "Password = pes; Database = pmc; "))
    {
        conn.Open();
        using (var command = new NpgsqlCommand("SELECT * FROM deposit.get_balance(@AccountNumber);", conn))
        {
            command.Parameters.AddWithValue("@AccountNumber", AccountNumber);
            using (var dr = command.ExecuteReader())
            {
                if(dr.HasRows && dr.Read())
                {
                    result = new AccountBalance
                    {
                        AccountNumber = AccountNumber,
                        Balance = dr["get_balance"].ToString(),
                    };
                }
            }
        }
        return result;
    }
}

Everthing is working as of now and I am getting the expected result

{ "accountNumber": "RD-0000050", "balance": "1100" }

What I have learned about executeScalar is that it only returns the value from the first column of the first row of your query. And my query returns only one column and i did tried using executeScalar going through some tuts but alas, didn't get that working. How do I use executeScalar in this scenario? Any detailed link, explanation would be helpful.