"according to the mohar of the virgins": What do we learn from "the mohar of the virgins." This (seemingly) comes to teach, but it emerges as learning, i.e., just as there, (Devarim 22:29) fifty silver, so, here, fifty silver.
Instead, It Confirms and Receives Its Definition from the Parallel Law
Mekhilta Tractate Nezikin 17:13