Is it permissible for my husband to sell the leftover road-paving material that he obtained for free from a company official without the employer's knowledge, and is the money he received from the sale lawful?

The ruling concerning your husband's disposal of street remains depends on the permission of the authorized entity. If the official who granted him permission is authorized by the company, then his action is lawful. However, if the official is not authorized, then his action is impermissible, due to the Prophet's (peace be upon him) saying: "The wealth of a Muslim person is not lawful except with his willing consent."