Learn Finnish: Hän sanoi lisämaksun hyvitetyn eilen. - He said the extra charge was refunded yesterday.

It is the past passive participle of the verb hyvittää (to refund). The basic passive participle is hyvitetty (refunded). In this specific grammar structure (the referative construction), we add a genitive -n to the end, making it hyvitetyn. Following a verb like sanoi (said), it replaces the whole phrase "that [it] was refunded".

Why is the extra charge in the genitive case (lisämaksun)?

In this shortened sentence structure (the partisiippirakenne), the subject or object of the embedded clause is forced into the genitive (-n) case. Even though "extra charge" is receiving the action, this structure requires it to be lisämaksun to link it grammatically to the participle hyvitetyn.

Can I just use an että (that) clause instead of this structure?

Yes, absolutely! This sentence means the exact same thing as Hän sanoi, että lisämaksu hyvitettiin eilen. The että version is much more common in spoken Finnish, while the -tun/-tyn structure used here is mostly found in formal writing, news reports, and professional emails.

If hyvitetyn means it was refunded, how would I say "He said he refunded the extra charge" using this same structure?