I asked a helpful とちゃき at librejp and they didn't totally know what this line meant either:

>月２皮ってお塩きかしら

But they said it since the game is about a sex change, the "kawa" part is probably a clever pun on the "skinsuit fetish"

"(おそらくこれのパロディだけど
たぶん「皮」ってのが注目するところだと思う
皮モノっていうエロジャンルがあって、これも性転換をテーマにしてるジャンルだからあえて原作の「代わって」を「皮って」へ変えていると思う)"

Well, his explanation kind of helps. I don't want to overthink it but he has confirmed there is probably a kanji　substitution pun, and I think there might be a triple kanji pun here.

月２皮ってお塩きかしら

[In two months], the skin could be spanked/punished [salt]?

"In two month's that'll be replaced, I wonder if there will be punishment?"