Is it obligatory to repeat the prayer if prostatic fluid (madhy) is discharged after performing it, given that it resulted from foreplay with the wife before the prayer and purification from it was performed before the prayer?

If you washed the madhy (pre-seminal fluid), performed ablution (wudu), and prayed, then found more madhy after the prayer, your prayer is valid. This is because an event (hadath (definition)) is attributed to the nearest time it could have occurred. The madhy is considered to have exited after the prayer; therefore, your prayer is valid and does not need to be repeated.