Is what the questioner said, concerning leaving a girl after his recovery from an illness, considered a promise or a vow, given that he married and returned to her after two years, and what is the expiation for that, as he does not recall the exact wording?

Whoever makes a covenant with Allah to abandon a sin and then commits it, repentance is incumbent upon him for committing it, and an expiation of an oath for breaking his covenant; because a covenant is equivalent to an oath. This covenant is not considered a binding vow (nadhr) because it is a vow to abandon something forbidden.