Hi,
As you must be aware, you need to have the rebate procedure set up and have all the settings ready.
Yes! If you are wanting rebate based on group of materials and not only one, then you would need to use the Group Condition Check for the Rebate Condition Type for %.
Also make sure that you check with the Business on whether they want to give other upfront discounts in this scenario apart from rebate ? If yes, leave them as is.
I do not have the SAP System in front of me so cannot give the exact place in the Pricing Procedure where you need to assign the rebate.
Check Standard Pricing Procedure for your country[India I guess - excise is the key here] and find the condition types for rebate there and use the same in yours. Normally they should be under the Discount and Surcharge Bucket in Pricing. Else check the standard SAP Pricing Procedure - RVAA01[good old one] .
You might want to create a Group Condition Routine for the same. You can wait for this. First set up basic stuff and test.
Get back to me if you need more help.
Happy Rebating 
Thanks.