A knas (penalty) is never paid through the admission of the one who would pay it. For example, if someone steals $1 he has to pay $2 back, but if he returns the money by himself, he only pays $1 back, because the extra $1 is a knas.
In the last parshah (Naso), we find (5:7) a case where someone steals from a ger and he dies and has no inheritors. In that case, he pays the money to the kohen and adds a fifth to it.
However, this is what the ibn Ezra writes there:
"And he will add a fifth to it," if he admits from himself; but if there are witnesses against him [and he pays it back because of them], he adds two fifths.
If so, the first fifth is not a knas, because he pays it through his own admission; but the second fifth is a knas.
However, in the Gemara (very end of Makos 2b), they ask whether eidim zomemim is a knas. "Rav Nachman says: Know [that eidim zomemin is a knas], because the money is in the hands of the owners and they still pay." It implies that whenever the money is in the hands of the owners [i.e. they've been recompensed for any actual damages] and you still pay, it is a knas. However, in the above case in Naso, the money is in the hands of the owners and they pay, but the first fifth isn't a knas!
Can anyone resolve this apparent problem?