@Creature:
If you do not wish to sell at 2.5K and you think the "buyer" would still be interested even raising the price on him/her then i would say go for it.
Email him/her back and say you have made an error in your estimation and you where not expereinced enough to place a value on it.
You have consulted a partner with years of experience in the domaining industry and you have to raise your price with a certain percentage.
It's your right to place your own price on the domain and it's your right to cancell the sale in the proceedings of this sale how it took place.
No contracts or whatso ever.
Just your wriiten word in a email stating you are prepared to sell for 2.5k...
This is not sufficient enough to win a legal dispute. You can back out any time if you do not wish to sell unless you have commited to a contract by agreeing to certain terms and conditions.
And even in real life this can be aplicable,
Using Crooky's example...
-Imagine a young car sales man being asked how much a certain car costs.
-He answers 2.5K
-Customer agrees to buy and pulls out a wad of cash, a lot more then 2.5K
-Young car sales man tells the buyer please hold and wait here for a minute and i will return with some paperwork.
-In the meantime he tells his employer about the buyer and his wad of cash.
-The employer instructs the young sales man to tell the buyer to come to his office.
-The employer tells the buyer due to the lack of experience and the young sales man being misinformed about the details of the car and the expected sales price he has to mark up the sales price with a x amount percentage.
-Ending up the buyer A still buying the car or B being dissapointed and moving on.
Can this be done in real life? Yes it can but obviously you can't mention this directly to the buyer in the sense of "I did not know you had so much money, so now you have to pay more!" Of course this is absurd but with a twist like above it is applicable.
But is it acceptable? I think the majority would say "Thanks but no thanks" and will move on to find another car of the same type from another sales man since he/she feels like being scammed or just (if the thought of being scammed doesn't cross their mind) thinks the price is too high.
Now a domain is unique regarding keywords and extension so this is a advantage unless the buyer thinks he has viable alternatives.
This could be your pitfall! If the buyer really wants it and you think the buyer can afford it easily then you could mark up the price. Just don't over do it!
Regarding the USA residence requirements is also something you should think about!
Maybe they don't have a USA business presence or person working for their organization.
And it would mean if they would find out they would drop it from their side.
Remember a email is not a contract. At least from the point of view in my own country this is not something i should worry about.
I don't think a email with an agreement to sell something is regarded as a legal obligation.
I would find that hard to believe actually.
So ask yourself the following questions:
Is there a chance another buyer will come along paying the same exact price or even more in a time period you find acceptable?
Is your answer No?
Then Sell it!!
Is your answer Yes?
Then take the chance! And mark up your price and restart negotiations.
Just don't start asking $xx.xxx figures!
But to answer your innitial question (And this my own personal opinion)...you can withdraw your sale and say there has been a miscommunication with your partner(s).
Of course at your own risk of losing the sale entirely.
People don't like to be dissapointed.
Because of this i would say sell it and experience this matter as a lesson learned. They made a mistake but you did not act upon it...your mistake

.
If you act upon it now...you risk loosing the sale.
It's a calculated decision that only you can fully calculate regarding the domain and company in question.
Good luck with your proceedings.