The definitions of surmarque and démarque connue/inconnue in the glossary you link to have nothing to do with markup and markdown. Those terms are about price with respect to cost and original price. The French terms are being used to describe *inventory adjustments*; see in
http://www.retailsystems.com/retail-pos-dictionary.cfm#s. Démarque inconnue corresponds to (the value of) shrinkage; démarque connue, to breakage (and similar); and surmarque would therefore include a laundry list of *positive* inventory adjustments. "Negative shrinkage" gets some relevant Ghits and may be the best match.