Can you exit a voluntary fast before completion?

al-maawardii said:

When a person begins a voluntary fast or a voluntary prayer, then it is mustahabb for him to complete it.

If he exits from it before completing it, then that is permissible.

He does not have to make it up regardless of if he exited from it due to a valid excuse or due to no excuse at all.

Among the Companions, this is the view of

Umar (may Allaah be pleased with him)
Ali (may Allaah be pleased with him)
Ibn Umar (may Allaah be pleased with him)
Ibn Abbaas (may Allaah be pleased with him)
Ibn Mas'uud (may Allaah be pleased with him)

Among the Jurists, this is the view of

al-Thawri
Ahmad
IshaaQ

Maalik said that it is permissible to exit from it for a valid excuse and he does not have to make it up. However, if he exits from it without an excuse then he must make it up.

Abu Hanifah said that he has to complete it. If he exists from it prior to completing it, then he has to make it up regardless of if he exited due to an excuse or not.

al-Haawii al-kabiir
Volume 3 Page 468

The correct view is the first view. It is the view of the Companions (may Allaah be pleased with them). Who knows the Diin better than the Companions of the Messenger of Allaah (sallallaahu alayhi wa sallam)