E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
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.