Cost Adjustment Serial Number

Hii all,,

i want ask, how to change cost on adjust serial number,

for example item AA_Disc_SG_AAA_001 when purchase with Serial 200324 have price 1000 and we want to change to 900

im already use inventory adjsument -1 for this item, and make new inventory adjusment +1 with cost 900

and next im try to fulfill this item , and cogs its 1000

but system override my inventory adjusment and fulfill

before inventory adjusment -1 @1000 override to -1 @900

before fulfill 1 @900 override to 1 @1000

is this cz my setup inventory costing preference?

this my setup inventory costing preference