Is ghusl obligatory and does Maghrib prayer need to be repeated if a dry trace of semen is found on the underwear after urinating, knowing that the arousal occurred before Asr prayer and the trace was not discovered until the time of Maghrib prayer?

Impurity on clothes is considered to have occurred after the Asr prayer, because the general rule is to attribute an event to its closest time.

If the impurity is semen, ghusl (ritual bath) and repeating the Maghrib prayer are obligatory. If it is pre-seminal fluid (madhiy), then only washing the male organ and performing ablution (wudu') are obligatory.

In case of doubt between semen and pre-seminal fluid (madhiy), ghusl and repeating the Maghrib prayer are obligatory.