All perfect praise be to Allah, The Lord of The Worlds, and may His peace and blessings be upon Prophet Mohammad and upon all his family and companions.
It is permissible to lay down regulations for such funds, and the condition mentioned in the above question doesn`t involve any violation of Shariah and thus it must be abided by since the Prophet(PBUH) said, "Muslims are on (i.e. stick to) their conditions."{Related by Al-Hakim}. And Allah knows best.