Is it permissible, Islamically, to demand from a partner half of the loss? And is this transaction void, especially since the partner was aware that the shop was losing money and not worth the price paid for it? And is it permissible to follow American law, which waives the rights of creditors upon declaration of bankruptcy?

Concealing a defect in merchandise and deceiving the buyer is Haram (definition) (forbidden) in Sharia (definition), and the deceived party has the option (Khiyar) to either annul the contract or accept the defect.

This issue falls under the jurisdiction of the Sharia judiciary. If it is unavailable, one can resort to trusted Islamic centers for arbitration.

Declaring bankruptcy does not absolve the bankrupt person of their debts; they remain liable. Their assets can be seized if creditors demand it.

Any law that cancels the debts of a bankrupt person contradicts Sharia, unless the creditors voluntarily absolve the bankrupt person of their debts.