The Bible has much to say about rewards as motives in the Christian's life. To be sure, we are going to leave everything else behind when we leave here. The rewards, we are told, are treasures and trophies. What a tragedy it will be when many will have no trophies to lay at Jesus feet. We are told to lay up our treasures in Heaven.
Rewards are a very valid motive to witness.
God sent me to pastor a church where they had started a new auditorium, built it half way, and ran out of money. Like the Bible says, it became a scoffing. In a year's time our church had led seven contractors to Christ and an eighth one had come to our church already saved. We called a building committee with eight contractors on it. They jumped in, finished the building, and saved us the equivalent of a million dollars at today's market.