hi,
you can proceed with as Mr.Bijay suggested or
config with partial invoicing plan, but make sure that your PO should have account assignment
-Create PO with account assignment
-Click on invoice tab at item detail choose invoice plan --> partial invoice plan
-Maintain the dates & details for the payments
also refer following link it might be useful in your scenario;
http://wiki.sdn.sap.com/wiki/display/ERPSCM/MM-SRV+The+Invoicing+Plan
Thanks,
K J