War movie with a bittersweet ending with a guy sending the protagonist money

I’m not sure if this was a WW1 or WW2 movie, but as far as I recall there was this one scene where the protagonist was saying “I’ll bet you 400 dollars that you’ll return to your girlfriend after this and you’ll have a family together” and then at the end he says he never heard from him again except for money he received in the mail every month up until it amounted up to 400 dollars, and then nothing ever again. God please somebody help.