ssh - sftp/fish over double hop with different users

I am trying to set a ssh session over a double hop with different users on each hop to transfer files easily via fish/sftp.

my .ssh/config look like

Host middle_server
    User            foo
    Hostname        192.168.xx.xx

Host www.server.org target
    User            bar 
    HostName        www.server.org
    ProxyCommand    ssh middle_server nc %h %p 2> /dev/null

The connection to middle server is secure by rsa key, and direct from the middle server to the target with the bar user.

When I try to connect to the target, I got the following error:

[foo@localhost]$ ssh target
Enter passphrase for key '/home/foo/.ssh/id_rsa':
Permission denied (publickey,gssapi-keyex,gssapi-with-mic).

I can connect with the 2 following methods, so I guess the bar user is not used for the second hop, but I have no idea of what can be the cause.

[foo@localhost]$ ssh middle_server
Enter passphrase for key '/home/foo/.ssh/id_rsa':
[foo@middle_server ~]$ ssh bar@www.server.org
[bar@www ~]$ 

or

[foo@localhost]$ ssh -A -t foo@middle_server ssh -A bar@www.server.org
Enter passphrase for key '/home/foo/.ssh/id_rsa':
[bar@www ~]$

Any help will be greatly appreciated, thanks in advance!