Abu Dawud said: I heard Ahmad (b. Hanbal) permitting to make atonement before breaking the oath.
In-book : Book 22, Hadith 36
English translation : Book 21, Hadith 3271
22. Oaths and Vows (Kitab Al-Aiman Wa Al-Nudhur)
باب الرَّجُلِ يُكَفِّرُ قَبْلَ أَنْ يَحْنَثَ