Are Christians supposed to give out of legal obligation?
No. Believers give under grace, not by legal binding or legal ramifications. Giving is as God leads, as He has prospered each person, and as the Lord lays it on the heart—not according to a set of rules and regulations. They first gave themselves to the Lord.
Les explains that giving is not to be done under a legal system or with legal ramifications that demand a certain amount. The Lord expects the saints to give as He leads them, and under grace they give as God has prospered them and as the Lord lays it on their heart, not according to a set of rules and regulations. The example is that they first gave themselves to the Lord, stepping into God's saving grace, and then opened themselves to the Apostle Paul and his ministry. Titus was also involved in finishing the same grace among them, gathering offerings for the poor saints.