Is opening the water tap for several minutes to cool it considered extravagance? What is the ruling on touching the hand of a foreign woman when exchanging money? And is throwing away leftover food that is not suitable for the next day considered extravagance?

Opening the water tap to cool it down is not considered extravagance if it is necessary for ablution, drinking, and similar purposes. Otherwise, it is extravagance, and extravagance is spending beyond what is needed. The Prophet, peace and blessings be upon him, forbade extravagance in ablution even if a person was by a flowing river.

As for touching the hand of a foreign woman, it is forbidden, and hand contact is not necessary in business dealings.

If there is excess food, it is best to give it to the poor, then to animals. If neither is available and it cannot be stored, there is no harm in discarding it.