Mujahid (may Allah have mercy on him) said regarding this verse: It is a man saying to a woman during her waiting period, "You are beautiful, and I like you," while he keeps the proposal of marriage hidden and does not disclose it. All of this is permissible. "But do not make promises to them secretly" [Surah al-Baqarah: 235]: do not give a secret promise, meaning that a man says to her, "Do not go ahead and marry on your own, for I will marry you"; this is not lawful.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 14022