Riddle: What halacha is learned from something said by Satan?

Riddle: Name a halacha derived from a statement of Satan.

Answer

The statement is Job 2:4

וַיַּעַן הַשָּׂטָן אֶת יְהוָה, וַיֹּאמַר: עוֹר בְּעַד עוֹר, וְכֹל אֲשֶׁר לָאִישׁ--יִתֵּן, בְּעַד נַפְשׁוֹ

And the Satan said to God: skin on behalf of skin, and all a man has, he will give for his own life.

The Halacha is that while we may not redeem a captive if the price is too high, an individual is entitled to spend as much of his own money as he likes on redeeming himself (and possibly his spouse). This is based on the above verse. --Tosfos, Kesubos 52a, s.v. והיו.

My understanding of Tosfos is that the verse is describing a reality of human behavior. Since:

• Any normal person would do anything to save themselves

AND

THEREFORE:

• The rabbis excepted one's self from the "no exorbitant ransom" rule