Answered by Shaykh Faraz Rabbani
Is it permissible to pray a salah other than Salat-ul-Asr at the time of Salat-ul-Asr? If so, is it permissible to pray the salah at anytime during Salat-ul-Asr or is there a prescribed time?
Walaikum assalam wa rahmatullah,
I pray this finds you in the best of health and spirits.
 Before performing the obligatory Asr prayer, one can perform any prayers–whether voluntary or obligatory make ups.
 After the obligatory Asr prayer, it becomes prohibitively disliked and sinful to perform any voluntary prayers (even those with a reason, such as istikhara or greeting the mosque), until Maghrib time.
 However, it remains permitted to perform obligatory make up (qada) prayers after the obligatory Asr prayer, right until the disliked time of Asr enters. [ Shurunbulali, Maraqi al-Falah ]
Please search SunniPath Answers (http://qa.sunnipath.com) for related answers. Search for: disliked times of prayer.
And Allah alone gives success.