Our company selling smartphone on installment basis,we procure a stock on bulk but if the prices of smartphone is down then we receive a rebate from supplier-treatment?

I can help you record the rebates you've received from your supplier, Abdul. 

Rebates you receive from your supplier can be entered as supplier credit or credit card credit. 

Here's how: 

1. Go to the +New icon.
2. Choose Supplier credit or Credit card credit. 
3. Fill out the necessary details.
4. Once done, click Save and close. 

After this, you'll have to create an item for rebates and use them in the transactions. 

Furthermore, you might want to check this article and learn how to enter a prior balance for a customer who owes you money or a supplier you need to pay: Manage outstanding balances for customers and suppliers in QuickBooks Online. 

I'm always available if you need additional assistance managing your rebates in QuickBooks Online. I'll keep an eye out for it. Stay safe.