Paul says that the good work that God started in him God will finish it. The reason he has confidence is because God said He would complete it and if God says it God will do it.
Besides financial support, we could give prayer support or write letters of encouragement. When we give money to support a missionary we are supporting God's work.
Being a slave means that your committed to your master for life same with a saint of Jesus once your his your committed to him for life or you should be.