Does God support bargaining practices and vows?

Hannah makes a vow to give her future baby to God if she becomes pregnant. Does God answer prayers based on vows like this?

God doesn’t answer prayers based on our vows for him, but based on his own will (1 Jn. 5:14-15). Hannah wasn’t asking for a son for her own purpose, but for God’s purposes. For instance, if a person vowed, “God, give me a brand new car, and I’ll promise to follow you forever!” God wouldn’t answer this. However, if someone said, “God, give me a reliable car, so I can serve in a high school Bible study on the weekends.” This would be different. If you’re vowing to give what you get to God’s purposes, this would be different. Like Hannah, we would be asking for something, so that we could put it to God’s purposes—not our own.

Finally, we know that God doesn’t answer prayers based on our vows, because God gave Hannah five kids—not just one—later on (1 Sam. 2:21). This shows that God answers prayers beyond what we have in mind—based on grace—not vows (Eph. 3:20).