When someone says "Inshallah," which means "If God wills it will happen," there is no fixed response to the word. However, it is appropriate to acknowledge the sentiment by nodding in agreement, smiling, or saying "Inshallah" back. Another option is to respond with a simple "thank you". It is important to note that "Inshallah" is meant to be used seriously, when you genuinely hope that something will come to pass.