Is it obligatory to fulfill a promise made to Allah to perform the regular Sunnah prayers, and is abandoning them considered a sin?

The jurists differed on whether a promise of a devotional act is considered a vow or not. The more preponderant opinion is that if the person intended it merely as a promise, it is a promise and not a binding vow. However, if they intended it as a vow, then it is a binding vow that must be fulfilled. Accordingly, the questioner's statement, "I promised Allah that I would pray the regular Sunnah (definition) prayers with the intention that it be a promise," does not obligate her to fulfill it, and she incurs no sin by abandoning it. However, abandoning emphasized Sunnah prayers is not appropriate, as it may lead to negligence in obligatory duties and acts, which the Sunnah prayers compensate for any deficiencies in. Furthermore, persistent abandonment of Sunnah and supererogatory prayers can invalidate one's testimony.