apply sum function after group by extract month date

Readers

How to apply a sum function to rows grouped by date?

create table testgroupsum (itemname varchar(50),date date,amount money);
set datestyle to DMY;
insert into testgroupsum (itemname,date,amount) values ('item
1','2013-01-01','100');
insert into testgroupsum (itemname,date,amount) values ('item
2','2013-01-10','35');
insert into testgroupsum (itemname,date,amount) values ('item
3','2013-02-20','50');
insert into testgroupsum (itemname,date,amount) values ('item
a','2013-01-25','-500');
insert into testgroupsum (itemname,date,amount) values ('item
b','2013-02-28','-20');
drop table testgroupsum;

How to change currency unit for the data type money, i.e. to change
the currency sign from dollar ($) shown in a query result?

The manual states that date data can be extracted, so the following was tried:

select extract (month from (select date from testgroupsum where date >
'2013-01-01'));
ERROR: more than one row returned by a subquery used as an expression

The objective is to be able to apply sum function to items 1 and 2,
grouped by month value = 1. Then to apply a subsequent _net_ sum 1
items 1 and 2, item a, i.e. for month value = 1, compute sum
100,35,-500; month value = 2, compute sum 50,-20; etc.

Thanks in advance.