Cutting the nails of hand or foot, even cutting only one or some of the nails is Haraam for a Muhrim, but if the nail is damaged and its presence causes harm or severe discomfort then it can be cut.
Whenever cutting the nail occurs because of forgetfulness or ignorance about the matter, then nothing is on the Muhrim, but if it is performed deliberately its atonement is one portion of food (750 gr) for each nail and if he/she cuts all of the nails then the atonement is one sheep. If the Muhrim cuts the nails of the hands and feet in one session then the atonement is one sheep, but if this happens in two sessions then the atonement is two sheep.
There is no difference between cutting the nails by scissors, nail cutter or even by tooth.
A Muhrim can cut the nail in case of necessity, but it is an obligatory precaution to pay the same atonement.
Whenever a Muhrim cuts his/her nail according to the fatwa of a person (or a quoting by another person) in the state of Ihram and blood sheds, then paying the atonement of one sheep is obligatory for the person who has issued the fatwa and even if blood does not shed then, it is still a precaution to pay the atonement.