Following the Liberation of cities in Southern Germany (Stuutgart, Sigmaringen, Ravensburg etc...) by the 1st and 2nd French Armor Divisions, several German stamps have been overprinted . Those overprints, though not officiallty recognised by French postal authorities, have sometimes been used on mail by soldiers and cvilians and are seeked for by collectors. Sigmaringen overprints. Ravensburg occupation Stuttgart Overprints on Hitler stamps.
I'm afraid I don't have these particular ones (they are very rare) . The price would be in the Mayer catalogue mentioned under the stamps http://www.philatelie50.com/boutique/materiel-toutes-collections/les-catalogues/catalogues-philateliques-de-cotation-des-timbres/france-monaco-et-dom-tom/9896-catalogue-mayer-2015-timbres-de-la-liberation.html