How have the courts interpreted the meaning of a voluntary payment in a personal injury case?

In Morgan v. Palmer (1824), 2 B. & C. 729, Abbott, C.J., said: “It has been well argued that the payment having been voluntary it cannot be recovered back in an action for money had and received. I agree that such a consequence would have followed had the parties been on equal terms. But if one party has the power of saying to the other, ‘That which you require shall not be done except upon the conditions which I choose to impose,’ no person can contend that they stood upon anything like an equal footing.”