1 Corinthians 9
Paul's only concern is to be rewarded by God.
He knows he is free and is not obligated to anyone here on earth, however he is following through with a task that was entrusted to him by Jesus, and that is to preach the Gospel of Christ.
He works as hard as he can so he might save some, and ultimately receiving what is due him as promised by Jesus in Heaven.
To God be the Glory.