fat is in the fire

事情已严重到难以挽回的地步；生米已成熟饭；事情搞糟了；闯下大祸

Sorry, dear. The fat's in the fire. No way can you retract the "offer" without doing a lot of damage to the relationship.
对不起，亲爱的，生米已成熟饭，你不可能在不伤及双方感情的情况下取消答应人家的事。