What will happen if i will give negative review to buyer?
As you haven’t had any negative reviews from your buyers it would seem a bit strange if you were to give a negative feedback in return.
One buyer give me 3.5 stars even after taking extra work
I know it’s disappointing but 3.5 isn’t a negative, and they did explain why they left it.
Perhaps you can respond to the feedback explaining your side of things?
As a buyer, I hate to see sellers leaving negative reviews for buyers - it has no effect on their ability to use the site in the future anyway - it can just end up making you look vindictive and bad tempered.
Don’t render evil for evil. Be the bigger person, as @offlinehelpers suggested just explain what happened.
Probably nothing happens, treat it as a way to show the world you are good. The buyer does not care about the feedback, your next buyer cares what you say and how you say it. Smash a vase, swallow your anger, put up your best smile, and type: “It was a pleasure working with you”.
If possible, block that user, you never want to work for him more…
^^^^^ This is what you should do.
I noticed this occurs in most cases when I’m doing some extra work for free. Especially when buyer is more demanding / buyer don’t realize (or pretend?) that his requirements are too high in relation to the order extrass / my offer… and I’m doing that extra work for free anyway.
I recommend you to talk kindly about the details as much as possible.