The statement "when Christians give freely, God will supply" is based on a biblical principle that can be found in 2 Corinthians 9:6-8. This principle emphasizes that when Christians give generously and cheerfully to help others, God will provide for their needs in return.
This does not mean that God will only supply the needs of those who give or that their needs will be based on the measure they give. Rather, it means that God is faithful and will provide for the needs of His people according to His riches in glory. The provision may come in different ways, and it may not always be immediate or in the form that we expect, but God promises to take care of His people.
In essence, Christians are called to trust God and be generous with what they have, knowing that their giving will not go unnoticed or unappreciated. They can have the confidence that God will provide for them day by day as they seek to live out their faith and love others in practical ways.