The incident regarding Umm Sulaym (radhiAllahu 'anha) and Abu Tal'ha (radhiAllahu 'anhu) took place before the Hijrah, whereas aayah 24 of Surah anNisaa', which specifically mentions the Mahr, was revealed several years after the Hijrah of RasulAllah (sallAllahu 'alayhi wa sallam).(2)
"As one of the conditions is the status of dowry itself; being lawful and having pecuniary value in terms of a commodity whose transaction is considered legal by Islamic law."(3)
يجوز جعل المصحف ذاته مهرا لكونه متمولا، ولا يجوز جعل العمل به مهرا لكون ذلك غير متمول.
However, women do have the right to waive their dowry if they choose to do so freely and without coercion. (5)
RasulAllah (sallAllahu 'alayhi wa sallam) gave all his wives a Mahr of monetary value. Dollar-to-dollar, the price depends upon currency etc. but it was, at the time, equivalent to 1487.5 grams of silver. (6)