Negative number will increase the cost of goods sold and hence reduces income for period reporting period.
Here is rationale:
Inventory change is the difference of value between last reported financials and current financials (quarterly or yearly). If the current period inventory is lower than last period inventory we have a negative change and number accordingly.
I spoke about cost of goods sold in this post. Please see the link
This is the cost of goods that has been sold by company/entity. Meaning against revenue number what was the cost required to produce the goods or services. Otherwise known as matching principle in accounting. In short mandatory requirement for Profit & Loss account.
One of the way calculating Cost of goods sold is adjust the cost of goods purchased or manufactured with changes in inventory. If the change in inventory is positive it means there is unsold item lying and reduced from cost of goods sold. For negative change in inventory it will be reverse that means sold more than previous period and hence added to cost of goods sold.
If Purchase is 100 and change in inventory is say positive 10 then cost of goods sold is 90.
if purchase is 100 and change in inventory is say negative 10 then cost of goods sold is 110.
Logic is all purchases should not be matched because there may oversold or undersold in comparison during the reporting period,
I use this way: opening stock of inventory+purchase of goods IS EQUAL TO cost of goods produced+ closing stock of inventory