Answer: by Shaykh Muhammad 'Umar Baazmool, instructor at Umm Al-Quraa University in Makkah
If you are not able to wear the izaar and ridaa' (the lower and upper garments) of ihraam without underwear, pants, or the likes, then it is permissible for you to wear underwear or pants, and you have to pay a fidyah (ransom).
You have a choice with this fidyah, and it is similar to that which a sick person pays, the one mentioned in Allaah's Statement,
"And do not shave your heads until your sacrificial animal reaches its place of sacrifice. And if one of you is sick or has an illness in his scalp (and he shaves because of it), then (he must pay) a fidyah of fasting (three days), charity (feeing six poor people), or slaughtering (a sheep)." 
So you have the choice to fast three days, feed six poor people, or slaugher a sheep. This is since the ruling on your wearing of something under your izaar is a kind of relaxing similar to the relaxing of the one who shaves his head (for relief).
You are similar in ruling to the one who has to wear a tib-baan, something like short pants or leg stockings.
So I say: You can wear this under your izaar, and upon you is a fidyah, the one mentioned in the Verse. It is that you choose between fasting three days, feeding six poor people, or slaughering a sheep and distributing its meat to the poor people of the Haram.
And all praise is due to Allaah for his bounties.
 the meaning of Soorah Al-Baqarah (2):196
Source: Translated from a cassette recording with the knowledge and permission of the shaykh, file no. AAMB043, dated 1423/8/17.
Taken from www.bakkah.net