Settings for back dated sales Invoice postings in SAP and its impact

Hi SAP Gurus,

My client has requirement for back dated Sale Invoice posting in the last financial year means before 31.03.2014. Requirement is to create the Sales order then Delivery, PGI and then Invoice in backdates on 31.03.2014. System is not allowing to post the invoices in back dates and FI & MM Periods are open.

please suggest the entire settings for the same. I also want to know what will be the impact of this after posting in back dates.

Its already May month started and we need to do the posting on 31st March. Please reply with possible solutions along with the settings.

Thanks in Advance...