Later ask that person to come up to help you and to bring their scriptures. Ask them if they would do something for you right there. Once you have their OK ask them to give you a dollar. Of course they will try leave to find one for you but say "no right here right now". You can mess with them for minute and then ask them to read 1 Nephi 3:7, where they will find the dollar.
Remind the class that the Lord's way is not always known to us up front that we must have faith that he provided a way for his work to be done. Remind the class that when your volunteer struggled about the money many of them may have been able to give you a dollar right then out of their wallet or purse, but that for your purpose it was important that only the volunteer provide the dollar. And that the lesson would have no meaning if someone else did it. "The Lord does not call the equipped he equips the called". This lesson can be used for: Faith in doing the Lord's work. Accepting callings in the Church. Sustaining leaders.
Can be adapted for: Over coming impossible obstacles.