How to zero out all negative numbers in a group-by T-SQL statement

You could use a CASE statement

SUM(CASE WHEN i.Quantity < 0 THEN 0 ELSE i.Quantity END)

Or a more obscure version

SUM(NULLIF(i.Quantity, -ABS(i.Quantity)))

or just exclude these rows altogether in the WHERE clause if they are not needed for any other purpose.

just filter out the ones you don't want...

WHERE quantity > 0