I talk to one iphone dealer he told purchasing monthly installment method you can purchase iTunes card for down payment is it true

No. The iTunes gift card is not an acceptable method of payment for purchasing Apple products. This sounds like a scam. Make sure you are ordering from the Apple Store, Apple.com or an Apple Authorized Retailer.

If you are purchasing a used iPhone, please read thru this article first.

The All Too Common SAD Reality of Buying … - Apple Community

No, iTunes Cards cannot be used for a down payment.

What type of gift card do I have? - Apple Support