Is it permissible, in Islamic law, to sell oneself as a slave to escape the calamity of usurious debt, believing that worldly servitude is lesser than the wrath and punishment of Allah in the Hereafter?

One must fear Allah in every action. It is impermissible for a Muslim to sell himself into slavery to rectify a mistake, because slavery is legislated only for a disbeliever who was captured in war, and Islam encourages the liberation of slaves.

You must turn to Allah with a sincere heart, for Allah is the One who holds the treasures of the heavens and the earth, and He says: (And whoever fears Allah – He will make for him a way out And will provide for him from where he does not expect).

Repentance from usury does not necessitate despair, for Allah said: (But if you repent, you may have your principal – [thus] you do not wrong and you are not wronged). So, if you have the usurer's principal, return it to him. If not, then all that is required of you is regret and a firm resolve not to return to it. The usurer's principal is a debt upon you; if you become able, return it to him.