Quantity & Value based Discount at Billing

Hi Experts

Please look the below case and suggest how we can do the configuration and settings as well.

we want to make sure that discount on qty will follow the procedure where one scheme says that Customer will get 3 Unit free once his all biiling will reach 100 Unit for a particular Period ,

Case 1: A Customer having 1st Invoice-10 pc   No Additional Qty

                                        2nd Invoice 30 Pc  No additional Qty

                                        3rd Invoice 50 Pc   No additional Qty 

                                       4th Invoice 15 Pc  but with additional 3 Qty bcoz he touch 100   qty   with some specific period  say March 1st to 15th March.

Case 2: B Customer Having Same Situation but all things convert into Amount rather Than Qty.

Rgds

Pankaj Agarwal