Is it permissible for me to benefit from an item I obtained at a wholesale price by registering it under another customer's account without their knowledge, and must I return it if it is not permissible?

It is impermissible to register your purchases under another customer's account to obtain a discount, as this involves lying and consuming wealth unlawfully. Allah Almighty says: (O you who have believed, do not consume one another's possessions wrongfully, but only [in lawful] trade by mutual consent.) An-Nisa/29. And the Messenger of Allah, peace and blessings be upon him, said: (Beware of lying, for lying leads to wickedness) - the Hadith (definition).

The permission of the company owner is required to sell an item at a reduced price, or at a wholesale price, or to add an item to another customer's account without their knowledge.

It is obligatory to return the item and remove it from the customer's account if possible, then purchase it at its full price or at a discounted price with the company owner's permission. If returning it is not possible, there is no harm in benefiting from the item, while seeking forgiveness for agreeing with the representative.