PostgreSQL - ERROR: Cannot cast type character varying to integer

Hello I´m new in the list, and I want to learn a lot about postgresql, now
I´m working in a database and I have this problem:

I have a table called historicoventas as follows (just the structure)

>  historicoventas
> (
>   codigopedido        varchar(20)
>   codigohospital         varchar(20)
>   fecha             varchar(15)
>   producto        varchar(20)
>   unidades        varchar(10)
>   costo             varchar(20)
> )
>
And I want to create a view based on that table whit this instruction:

> CREATE OR REPLACE VIEW vw_temphistoricoventas
> (Pedido, Fecha, Hospital, Producto, Unidades, Costo)
> AS
> SELECT codigoPedido , (substring( fecha from 1 for 4 ) || '-'
> || substring( fecha from 5 for 2 ) || '-'
> || substring( fecha from 7 for 2 ) ) AS fecha,
> codigoHospital , producto , sum( unidades::int4 ) AS unidades, sum(
> costo::float4 ) AS costo
> FROM historicoventas
> WHERE ((producto = '759928') OR (producto = '759936') OR
> (producto = '4002382') OR (producto = '4002580'))
> GROUP BY codigoPedido, fecha, codigoHospital, producto
> ORDER BY fecha;
But I always get this message:
> ERROR:  Cannot cast type character varying to integer
My developer workstation is under win xp and postgresql 8.0 but in
production server (external web hosting partner) I have a Linux Red Hat with
kernel 2.4.21-27.0.2.ELsmp y PostgreSQL 7.3.
In local machine all castings between data types works fine but in server it
doesn´t.
convertions like sum(unidades::int4)  works fine and reading the postgresql
7.3 docs I can see this type of convertions are enabled by the system
Can somebody help me with this topic?

Thanks a lot

>
> Mauricio Fernández A.
> Ingeniero de Sistemas
> Universidad Autónoma de Manizales
> mfacontacto@ono.com
> movil: 607365968
>
> P.D.  Some historicoventas table rows can be:
> codPedido    codHospital    fecha        producto    unidades
> costo
> 0001001299     100058         20050103     759936         1
> 2375
> 0001001299     100058         20050103     759928         1
> 2375
> 0001001300     100078         20050103     759936         1
> 2375
> 0001003044     100086         20050614     759936         1
> 2275.25
> 0001003128     100062         20050623     759936         2
> 4550.5
> 0001003129     100069         20050623     759936         2
> 4550.5
> 0001003131     100019         20050623     759928         5
> 11376.25
> 0001003131     100019         20050623     759936         3
> 6825.75
> 0001003132     100075         20050623     759936         4
> 9101
> 0001003133     100106         20050623     759936         2
> 4550.5
> 0001003134     100031         20050623     759936         2
> 4550.5
> 0001003135     100037         20050623     759936         3
> 6825.75
> 0001003136     100009         20050623     759936         5
> 11376.25
> 0001003137    100009         20050623     759936         1
> 2275.25
> 0001003138     100024         20050623     759936         4
> 9101