The Book by Herve Torchet: Accounts of the Duc of Bretagne 1420-1433, available through MEMODOC.fr, may provide the information you are seeking, if your ancestors bought nobility.
"Fouages" were a royalty or tax paid by household. Torchet has published several books about changes in the Fouages. One is for Saint-Malo where my ancestors are from: The Reformation of the Fouages of 1426, Diocese of Saint-Malo.
These "Fouages" books are quite expensive, 215 Euros, plus shipping.
Sue King Nusbaum
Longboat Key, FL, US