Hi Zaiedul Hoque,
First of all there is no need for a JOIN between MSEG and MKPF.
Then for you selection of DMBTR and MENGE pls do,
DATA: BEGIN OF it_temp,
dmbtr TYPE dmbtr,
menge TYPE menge,
END OF it_temp.
CASE mwart.
WHEN 101 OR 918 OR 919 OR 262 OR 602.
SELECT dmbtr menge FROM mseg INTO it_temp WHERE bwart = (101, 918, 919, 262, 602).
WHEN 102 OR 917 OR 920 OR 261 OR 601.
SELECT dmbtr menge FROM mseg INTO it_temp WHERE bwart = (102, 917, 920, 261, 601).
LOOP AT it_temp INTO wa_temp.
wa_temp-bmbtr = wa_temp-dmbtr * -1.
wa_temp-menge = wa_temp-menge * -1.
MODIFY it_tempFROM wa_temp INDEX sy-tabix.
ENDLOOP.
ENDCASE.
HOPE THIS HELPS. 
Happy Coding,
Santhosh Yadav