I think you can give your client access to the manager as a user with a non-administrator role (you have to create that role yourself). And then lock the chunks that the client is not allowed to edit with the "Only administrators can edit" option that can be set per chunk. Probably the client can see the code but not edit it.
Bummer. Guess that means in order to give a client access to chunks (for PayPal buy now buttons in this particular case) I have to give them Administrator privileges, which makes me nervous.