The Ketzos Hachoshen (424:1) explains that a bogeres or an orphan that has been seduced waives her rights to any payments. In truth, there is an obligation for the seducer to pay, but later, she waives the payments. This functions because of the concept of mechilah.
Reb Shimon Shkop (42) states that this cannot function based on an ordinary mechilah because the payments are not yet in existence; one cannot be mochel something that is not in existence yet. Furthermore, the girl should be able to retract from the mechilah before the seducer stands for judgment. Rather, she is waiving her rights to make any claim against him. The right to this claim is in existence and once she decides this, she cannot retract later.
The Ketzos (207:8) cites from the Bnei Yaakov that the argument that mechilah should not be valid because the payment is not in existence yet is not a valid one. This is because the seducer is liable for the payments as soon as he cohabits with her. Since she consented, she is mochel this obligation. He adds that even if she can be mochel the payments before they are actually in existence, she certainly cannot sell the rights to the payments to another before they are in existence. The payments are only in existence in respect to her rights to waive the obligation on the seducer to pay her.
The Yerushalmi disagrees with our Gemora and states that the girl who is seduced can only waive the rights to the embarrassment and depreciation payments, but not the fine. This is because a person is unable to be mochel something that is not yet in their possession. The Ridvaz explains: The primary purpose of the fine is not a monetary payment; it serves as an atonement for cohabiting with this girl, and she has no right to waive the payment that the seducer needs to pay in order to be forgiven.