All perfect praise is due to Allah, The Lord of The Worlds; and may His peace and blessings be upon Prophet Mohammad and upon all his family and companions.
A pilgrim is permitted to come out of the state of Ihram (minor Tahalull) on the Day of An-Nahir (Day of E`ed) once he performs two of the following: throwing the pebbles, cutting or trimming hair and Tawaf (circumambulation). A pilgrim who throws the pebbles and doesn`t perform any of the aforementioned rites along with it isn`t permitted to come out of the state of Ihram and wear form-fitting clothes, because he has violated one of the restrictions of Ihram; therefore, he is obliged to pay a ransom.
An-Nawawi (May Allah have mercy on him) said: "Minor Tahalull is achieved by performing two of the three rites of Hajj: throwing the pebbles and cutting the hair , throwing the pebbles and Tawaf or Tawaf and cutting the hair. Major Tahalull becomes effective once the pilgrim performs the remaining rites." {Al-Majmou`(8/229)}. Taking off the clothes of Ihram, while in the state of Ihram, out of ignorance or forgetfully isn`t considered a sin and so, that pilgrim isn`t liable to pay a ransom. This is indicated by the verse, " But there is no blame on you if ye make a mistake therein: (what counts is) the intention of your hearts: and God is Oft-Returning, Most Merciful." {Al-Ahzaab/5}.
Moreover, a pilgrim isn`t permitted to authorize another pilgrim to throw the pebbles on his behalf, save for a lawful excuse, such as being sick or in captivity. However, if there was no lawful excuse, throwing the pebbles by the authorized pilgrim doesn`t avail him, and he (the authorizer) isn`t absolved from the liability.
Al-Khateeb Ash-shirbini (May Allah have mercy on him) said: "A pilgrim, who isn`t able to throw the pebbles for an excuse that isn`t expected to end before the period for performing this rite ends, such as being sick or in captivity, is obliged to authorize another to throw them on his behalf……..It is conditioned that the deputized has already thrown pebbles for himself, if not, then the ones that he throws are considered his, not those of the pilgrim that authorized him." {Moghni Al-Mohtaj, 2/278}. And Allah knows best.