Jean-Michel, you’ll need to create a hack for this.
Make a copy of the file [hilite mono]/wp-content/plugins/s2member/includes/menu-pages/paypal-buttons.inc.php[/hilite] and put it in your [hilite mono]/wp-content/mu-plugins/[/hilite] directory. This is to prevent that future updates of the plugin remove your edit. You’ll have to check manually if new updates are supposed to do something to this file, so you have your version up-to-date with the new fixes as well.
Now, in line 75 you’d edit the $ symbol:
[hilite pre_code]
... I want to charge: $ ...
[/hilite]
If your client will be changing the currency, it may be best to just remove the currency symbol or he’ll get confused, cause the symbol won’t be updated in the button generator when the currency is.
In line 76, find the part that says:
[hilite pre_code]
' . trim (c_ws_plugin__s2member_utilities::evl (file_get_contents (dirname (dirname (__FILE__)) . "/templates/options/paypal-currencies.php"))) . '
[/hilite]
and replace it with everything starting from line 6 in the file [hilite mono]wp-content/plugins/s2member/includes/templates/options/paypal-currencies.php[/hilite].
There remove the [hilite mono]selected="selected"[/hilite] part from the USD option and put it in the EUR one.
I haven’t tested this, but I think it’ll work. I hope it helps. :)