Some years back people used to go to new places and collect money for charity (for schools, Masajid, etc) and the organizers covered the travel expenses which occur during this. The process continued and the people increased the demand from travel expenses to living expenses too.
Now the case is such that people are demanding a fixed percentage meaning say for example they collect 20,000$ then they ask say 15% of it. This thing is an open market and a open business these days so far that a auction is being conducted to achieve a minimum percentage. Is this allowed in Islam?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

There is no harm for charitable organizations to choose a qualified person to collect funds for them, to pay his travel expenses and fix wages for his work.  However, it is better for people working in such organizations to be volunteers and seek the reward from Allah, the Most-High.

The evidence for such people to take this money is that Allah has allowed the person collecting Zakah to take an amount of money for this work even if he is rich.  Collecting Zakah, without doubt, is a collective obligation because it is a part of establishing one of the pillars of Islam, which is Zakah.  And if it is permissible to take wages to achieve what is in this position, then it is permissible to take wages on something of a lesser degree than that, but it should not be a percentage of the collected money, because the amount that will be collected is not known.  It is possible that he collects a lot of money with very little effort and it is possible that he will not collect anything with any great effort.  Wages in this manner are not known and in the Islamic Shariah wages should be defined and fixed.

Allah knows best.