how to give product discount

Dear Sd experts

i have one doubt plz let me know all

if customer bought 10000 rs products ,he can take any item which is below 700 rs item for free of cost

ex : customer took 12000 rs stock then he will take 700 rs item  suppose if item cost is 750 rs remaining amount he is supposed to pay

12000 rs +50 rs : 12050

so here 700 rs should not add in net value

Plz tell me any sap stranded method is there 

I am very interested to know this scenario configuration bz recently i have gone to one store who are applying this business process

thanks a lot