Is it permissible to sell company merchandise at a price higher than what is requested by the company, and keep the difference?

If the company has set a maximum selling price, then you are not permitted to exceed it. However, if it has set a price but has not prohibited selling for more, then an increase is permissible. In both cases, the increase belongs to the company, not to the agent, because the agent acts on behalf of the principal. This is supported by the hadith of Urwah ibn al-Zubayr, who bought two sheep for the Prophet, peace be upon him, for one dinar, and sold one of them for one dinar. The Prophet, peace be upon him, then took the dinar and the (remaining) sheep. If the company agrees with you that any price increase after the sale will be yours, then it becomes lawful for you.