Brachos 31 – ברכות לא

Chana declared ‘I will seclude myself with a stranger in front of my husband’ which will result in my drinking the Sotah water.  How can she allow herself to transgress the prohibition of Yichud?

The Gemara says if one needs to fast on a Shabbos, he should nevertheless atone (for his neglecting Oneg Shabbos) by fasting again… If fasting on Shabbos requires atonement, how can one transgress a prohibition with the intention to atone for it later?