How to Sum multiple Column with multiple value with SQL?

I am looking for a solution to the following:

1. Go in to the users table and find a user who has listed items on the site. In this users table, there is no column about auctions. Instead, it is connected to an accounts table with a key (in accounts, this column is called user)

2. From these IDs (users which have listed items for auction), I need to find their account balance. This is also in the accounts table. The balance is contained in a column called operation_amount. I also have another column called operation_type which describes whether a user has a positive or negative balance. For example, if operation_type = 1, he has a negative balance, while if operation_type = 2, he has a positive balance.

Now I have another table called tmpinvoice where there is a column called amount. This shows how much in fees a user needs to pay to the site administrators.

Given this, I need to calculate how much he must pay in total. For example, if a user has a $200 balance, I need to check whether it's negative or positive based on the operation_type.

So i have querry where is Do this only for record

SELECT  u.id                    AS id_user, 
  u.nick,

    CASE ac.operation_type
    WHEN 1 THEN ac.operation_amount - tm.amount
    WHEN 2 THEN ac.operation_amount + tm.amount
        ELSE 'N/A'

        END             AS `fee`    


FROM auctionbg_search.accounts          AS ac   
    LEFT JOIN auctionbg_search.users    AS u    ON TRUE
        AND u.id = ac.user
    LEFT JOIN auctionbg_search.auctions AS a    ON TRUE
        AND a.id = ac.auction
    LEFT JOIN auctionbg_search.tmpinvoice   AS tm  ON TRUE  
                        WHERE TRUE
        AND tm.amount = ac.operation_amount

Here what result im reciving http://gyazo.com/3d7e7f52ee14d21cc8c8d33b6bbc479a

Yes but this calculate 'fee' only for 1 value in column , what if user have multiple values

like this user :

http://gyazo.com/c3bdb29fa235044ab888dc0385bbcdbd

I need calculate total amount from operation_amount of that given user and remove tmpinvoice from that total amount ,

1 my friend told me

to use IF(SUM(ac.operation_amount) , IS NULL ,0,sum(ac.operation_amount) and to join 2 time accounts(table) for both case with + and -

join 1 time for + , 2 time for -

but i cant figure out how will looks :)

Im using MySql 5