My buyer wants to modify order amount. What will affect more on my profile Cancelation or Bad review?
Its not possible to reduce the amount of an order.
Yes, you can increase but can not reduce.
If you want to increase the amount, offer a gig extra.
If you want to reduce the amount. Cancel the order and send a new one with the right price. The buyer should accept it, so why worry? Yes, your completion rate will drop a little, but it’s better than a bad review (especially if the reason was that you didn’t negotiate the price before sending an offer)