What is the ruling on extending an internet connection to neighbors and friends, provided there are conditions and procedures to ensure its lawful use, and what is the ruling on the money remaining after paying the subscription and purchasing the devices?

It is permissible to share an internet connection with others unless the contract explicitly prohibits it. However, it is not permissible if it is highly probable that they will use the internet for prohibited things, as this falls under cooperation in sin and transgression. The statement "We are not responsible for any misuse" does not absolve one of responsibility before Allah if it is highly probable that the use will be unlawful. The remaining money after the subscription is subject to the agreement: if it was an agency (wakalah), the surplus must be returned; if it was a lease (ijarah), there is no harm in taking it. The hadith "Whoever listens to a singing girl, molten lead will be poured into his ears on the Day of Resurrection" is invalid, and valid hadiths indicating the prohibition of singing suffice.