Is it permissible for a trainee to purchase devices other than a computer with the amount allocated to him by the company, then submit an invoice for the full amount for a computer, while committing to return only the computer upon request?

It is not permissible for the trainee to dispose of the amount allocated for purchasing a computer except by purchasing a computer within the limits of that amount. The trainee is not the owner of the amount, but rather an agent. Whoever violates this by purchasing a computer for less than the allocated amount and benefiting from the remainder for themselves, or keeping it and submitting an incorrect invoice, has cheated and exceeded the permission of their principal. The company has the right to reclaim the full amount, or the purchased devices, or take the device and ensure the withheld amount.