It is legal: there is no obligation to disclose the invoices or sales figures unless--1) Your commission agreement or other employment contract specifically says that the employer must do this; or2) You file a lawsuit--such as on the grounds that you believe you are not being paid the amount to which you are entitled--in which case, you could use the legal process or procedures of "discovery" (such a written interrogatories or questions; document production requests) to get access to this information.