These two are very different stories. Probably the biggest similarity of the two is that both stories are centered around a lie. Mme. Loisel lies to her friend Mme. Forestier when she doesn't tell her she lost the necklace, then puts herself into debt for ten years in order to give her back a real replacement only to find out the original was a fake. In Ambushthe narrator's daughter knows her father writes war stories and asks him if he's ever killed anyone. He lies to her and resolves to one day tell her the truth, but for now allows her to believe that he has not killed anyone. The truth is he did kill a man he wanted to scare away with a grenade in Vietnam and he has vivid memories of the corpse with a hole where an eye should be. Both Mme. Loisel and the narrator in Ambush live with regret. One lives with the regret over lying out of shame about a necklace the other lives with regret over killing someone, possibly needlessly, and then lying to his daughter out of shame.
The stories plots are radically different, The Necklace is about a spoiled ungrateful young woman who learns her lesson the hard way. She attends a ball and must have an expensive dress and jewelry as well which she loses and her pride keeps her from telling the truth and puts her into debt. Ambush is about a man who writes war stories and is conflicted over having killed someone in the war, even if it might have been an enemy, he struggles with his past.