I booked ticks from SFO to BKK, used the onward leg and now I want to post pone the return journey by a week.
The tickets were booked using vayama thru skyscanner.
When I called cathay, the change fee and fare difference made it 4000-9000$.
(This is business class. Total coast of ticket was 4700$)
Cathay rep did suggested that I cancel my ticket and just book economy. She also put a hold on the ticket for 1day.
When I called Vayama, they said it there is no partial refund as I had already used up part of the ticket.
Does US DOT or any agency have a policy. Can vayama make up their own rules?

Also, what happens if I don't show up for the flight.
I am not planning to do this, but I do want to know.

I can't speak for Vayama, but it's possible you're on some discount biz ticket and changing the return by a week forces a reprice into a much higher fare class. Full J on the route is around 7-10k USD I'd assume. $4,700 USD for a round-trip from SFO does sound like low to mid-priced ticket for the route.

It's also possible just the flights you asked for aren't available in your cheap fareclass, This would be a better situation for you. Maybe you can just see if a different flight or different day will give you the original ticket pricing.

If you don't show up for your next sector, the rest of the itinerary will cancel.

Lesson for the future, if you're spending multiple thousands or more on tickets, don't use discount online travel agents unless it's Amex, or a premium one you have a relationship with.

Sounds crappy.

Not really. The fare difference was 1000$ minimum from what I remember.
I was flexible with the dates and time and even class which they said they cannot change.
Absolutely.

You were booked on a very restricted ticket which should incur heavy penalties for changes and revised itinerary should price based on rebooking and current inventory.

unfortunately SFO is premium heavy. Don�t count on el cheapo pricing unless you are willing to wait for weeks and months

To say Vayama making up their own rules is a overkill.

I would say your best course of action now is to obtain the fare rule of your ticket. The fare rule pretty much defines everything.

In general, to minimize any fees related to changes, the same fare class has to be available first. So given your post contains virtually no useful information, you are basically on your own, unless you can provide further detail.

What may be happening is that they were repricing it to what you actually flew... a One way. Given that would almost certainly have cost more than your ticket, there is no value left to refund. My understanding is that such calculations are normal (otherwise people could avoid the high cost of one ways by cancelling the return)

This, with a further twist. In addition to the CX fare rules, Vayama may have a refund / change policy over and above what CX imposes.

To answer the question of whether Vayama can make up what it wants, it may not. Rather, OP chose to enter a contract with Vayama and those are the rules.

Rather than relying on what some random agent told you on the phone, I would spend the time to carefully review the fare rules and then Vayama's rules and figure out what is best and doable under the circumstances.

The result here is entirely possible and it may well be cheaper to purchase a new ticket back to the US in Y rather than pay for a change and reissue. But, no way to tell without a review.