Brewer's: Diomedean Swop
An exchange in which all the benefit is on one side. This proverbial expression is founded on an incident related by Homer in the Iliad. Glaucus recognises Diomed on the battle-field, and the friends change armour.
For Diomed's brass arms, of mean device, For which nine oxen paid (a vulgar price), He [Glaucus] gave his own, of gold divinely wrought, An hundred beeves the shining purchase bought.
Pope Iliad vi.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894