Is the difference between the two prices for the stagnant goods that I disposed of rightfully mine?

It is not permissible for an agent to take the price difference for himself without the employer's consent, because he is acting as an agent for the employer in selling the goods, and the employer must be informed of the actual price. If the employer agrees to the agent taking the difference, then there is no harm. Otherwise, it is not permissible for him to take it. This is supported by the Hadith of Urwah ibn al-Ja'd al-Bariqi: The Prophet, peace and blessings be upon him, gave him a dinar to buy a sheep for him. He bought two sheep with it, sold one for a dinar, and came to the Prophet, peace and blessings be upon him, with a dinar and a sheep. The Prophet supplicated for blessings upon him in his trade. If the increase had been the right of the Prophet, peace and blessings be upon him, Urwah would have given it to him.