Demo, Sql Server - rextester

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CREATE TABLE inventory2
    (
      timestamp DATETIME2 DEFAULT (getdate()) NOT NULL
    , Ord_label INT NOT NULL
    , Prod_id   INT NOT NULL
    , Loc_ID    NVARCHAR (3)
    , Price     INT NOT NULL
    , Buy       INT NOT NULL
    , Sell      INT NOT NULL)

INSERT INTO inventory2
VALUES  ('2017-02-22 12:24:00',30411,54,'72',52,2,0)

INSERT INTO inventory2
VALUES  ('2017-02-23 12:24:00',30412,54,'72',53,2,0)

INSERT INTO inventory2
VALUES  ('2017-02-24 12:24:00',30413,54,'72',55,0,3)

INSERT INTO inventory2
VALUES  ('2017-02-25 12:25:00',30414,56,'70',42,2,0)

INSERT INTO inventory2
VALUES  ('2017-02-25 12:34:00',30415,56,'70',32,2,0)

INSERT INTO inventory2
VALUES  ('2017-02-26 12:24:00',30416,56,'70',62,0,4)

INSERT INTO inventory2
VALUES  ('2017-02-27 13:34:00',30417,56,'77',2,2,0)

INSERT INTO inventory2
VALUES  ('2017-02-27 14:24:00',30418,56,'77',4,0,1)

INSERT INTO inventory2
VALUES  ('2017-02-27 14:25:00',30419,60,'80',4,0,1)

select case when sum(buy) > sum(sell) then max(case when buy > 0 then timestamp end) else max(case when sell > 0 then timestamp end) end as timestamp,
    case when sum(buy) > sum(sell) then max(case when buy > 0 then ord_label end) else max(case when sell > 0 then ord_label end) end as ord_label,
    prod_id,
    Loc_ID,
    case when sum(buy) > sum(sell) then max(case when buy > 0 then price end) else max(case when sell > 0 then price end) end as price,
    case when sum(buy) > sum(sell) then sum(buy) - sum(sell) else 0 end as buy,
    case when sum(sell) > sum(buy) then sum(sell) - sum(buy) else 0 end as sell
from inventory2
group by prod_id,
    Loc_ID
having sum(buy) - sum(sell) <> 0;

View schema
Execution time: 0,02 sec, rows selected: 3, rows affected: 9, absolute service time: 0,17 sec