Can i exchange my iphone 6s with iphone 7 by paying a small amount?

I think you need to ask Apple that but I very much doubt it.

Hi

Depending on your location, Apple may offer instant in-store credit or an Apple Store Gift Card when trading in an iPhone 6s.

To see examples of estimated trade-in values in the US, for example:

http://www.apple.com/iphone/trade-up/

You may also wish to consider the option of selling your iPhone 6s privately and putting the proceeds towards your new purchase.

I want to exchange my iphone 6s 64gb with iphone7 32 gb paying an amount of money. what should I do?

Goutamdas.6666 wrote:

I want to exchange my iphone 6s 64gb with iphone7 32 gb paying an amount of money. what should I do?

See my previous reply.