If you could post the data as tables, with CREATE TABLE and INSERT statement(s) -- rather than as a "splat" on the screen, which we have to convert to data to try to run a query, someone can code the query for you.
declare @INV1InvoiceLineItems table (Item varchar(20), Quantity int, Amount decimal(10,2))
insert into @INV1InvoiceLineItems select 'A1000' ,3, 300.00
insert into @INV1InvoiceLineItems select 'B101' ,4 ,600.00
insert into @INV1InvoiceLineItems select 'C101', 2 ,100.00
declare @OITMItemMasterData table (Item varchar(20), Property varchar(1))
insert into @OITMItemMasterData select 'A1000', 'Y'
insert into @OITMItemMasterData select 'B101', 'N'
insert into @OITMItemMasterData select 'C101', 'Y'
select * from @OINVInvoiceHeader a ,
( select
sum(case when Property = 'Y' then Amount else 0 end ) TotalPropertyY,
sum(case when Property = 'N' then Amount else 0 end ) TotalPropertyN
from
@OITMItemMasterData a join @INV1InvoiceLineItems b
on a.item = b.item ) b