Is it permissible to fast on that day, or is expiation (kaffarah) obligatory, and what is its amount, and what should I do, knowing that I broke my fast with sandwiches taken from a fast-food restaurant without the knowledge of those in charge and amidst work pressure?

Since the sandwiches were taken unjustly, the duty was not to eat from them and to advise your friend. Now, you must pay the value of what was eaten to the restaurant owner, advise your friend to do the same, and repent to Allah. The fast is valid, and there is no expiation other than repentance, seeking forgiveness, and returning the value.