I have been told that provided 2 rakaats of the Asr salaat have been completed before the Maghrib azaan commences, one can complete the Asr salaat and it will be valid.
The time for Asr Salaat is divided into two, Mustahabb (preferable) and Makrooh (reprehensible).
The Mustahabb time for Asr Salaat is until the sun is so bright that it is difficult to set one’s gaze on the sun, the eyes will dazzle. The Makrooh time commences upon the sun losing that brightness. Asr Salaat can be performed until sunset. Fajr Salaat can be performed until before sunrise.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai