First of all your IF EXISTS need corrections
<pre lang="SQL">
IF EXISTS ( SELECT Name FROM sysobjects
WHERE Name = 'TRI_deleteINSTOCK' AND Type = 'P' )
DROP trigger TRI_updateINSTOCK
GO
</pre>
it should be
<pre lang="SQL">
IF EXISTS ( SELECT Name FROM sysobjects
WHERE Name = 'TRI_updateINSTOCK' AND Type = 'P' )
DROP trigger TRI_updateINSTOCK
GO
</pre>
Trigger is working fine as defined there might be an understanding issue which is the following:
Let say Store A quantity exceed 50,000. this will be inserted in big store table.
what if someone check the records in the instock table and think the quantity entered is wrong and he/she edit the quantity, doing so the total quantity goes less then 50,000.
now where that store should be a big store or not ?