You have crossed your S (French). You have cheated me in your account; you have charged me pounds where you ought to have charged shillings, or shillings where you ought to have charged pence. In the old French accounts, f ( = s) stood for sous or pence, and f for francs. To cross your f meant therefore to turn it fraudulently into f.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894