Praise be to Allah.
If the company has stipulated the price and said that you should not sell it for more than that, then it is not permissible for you to sell it for more than the stipulated price. However, if the company has stipulated the price but has not said that you should not sell it for more than that, then it is permissible for you to sell it for more. In either case, the extra money belongs to the company and it is not permissible for you to take it.
That is because the agent should work for the interests of his employer, not for his own interests.
The evidence for that is the report narrated by al-Bukhaari (3643) from ‘Urwah, that the Prophet (peace and blessings of Allaah be upon him) gave hima dinar to buy a sheep for him. He bought two sheep and sold one for a dinar, and brought him a sheep and a dinar. The Prophet (peace and blessings of Allaah be upon him) prayed that he would be blessed in his transactions and that even if he bought dust he would profit from it.
‘Urwah was acting as the agent of the Prophet (peace and blessings of Allaah be upon him) in this purchase, and he was able to make a profit in his buying and selling. This profit belonged to the Prophet (peace and blessings of Allaah be upon him), because if it had belonged to ‘Urwah, the Prophet (peace and blessings of Allaah be upon him) would not have taken it.
Ibn ‘Abd al-Barr said:
There is no dispute among the scholars that it is permissible to act as an agent. But the scholars differed concerning the meaning of this hadeeth and whether, if the agent buys more than he was told to, that transaction is binding upon his employer. For example, if a man says to another, Buy me a pound of meat with this dirham, and describes what kind of meat it should be, and he buys four pounds of that kind of meat with that dirham. The opinion of Maalik and his companions is that he has to accept this transaction, if the meat is of the kind stipulated but is of a larger amount because he has done well. This hadeeth supports this view, and it is a jayyid hadeeth. It proves that the two sheep belonged to the Prophet (peace and blessings of Allaah be upon him), otherwise he could not have accepted the dinar or approved of this transaction.
The Standing Committee was asked about this matter and they said:
It is permissible to sell a product for more than its price if that is possible, but the extra money belongs to the owner of the product. But if the owner stipulated that it was not to be sold for a higher price, then it should be sold for the price stipulated by the owner.
Fataawa al-Lajnah al-Daa’imah, 13/96
But if the company stipulated the price to you, and agreed with you that if you sold it for more then the extra money would be yours, it is permissible for you to sell it for more and the extra money is yours.
Ibn Qudaamah (may Allaah have mercy on him) said in al-Mughni (7/361):
If he said, Sell this garment for ten and anything extra is yours, then that is valid and he is entitled to the extra money… Ibn ‘Abbaas did not see anything wrong with that.
And Allaah knows best.