by Jean Gabus (1958). It was a rare book here in the US and I had to get it on inter-library loan. I think there is a German translation.

Gabus visited Niger, Sudan, and Nigeria in 1942, then in 1946-7. In 1950-1 he was in Niger, Sudan, Nigeria, Senegal, Morocco, and Mauritania. In 1952-53 he visited Algeria, Niger and Mali.

There is only one short paragraph in the entire book that relates to amber, so I'll quote it:

L�ambre:
L�ambre, limit� � la Mauritanie, a des valeurs prophylactiques. Depuis longtemps d�j�, d�astucieux marchands syriens ont parcouru les march�s sahariens, les camps importants pour �changer les boules d�ambre authentique des Maures �meyyal� contre des imitations bien rondes et polies en mati�re synth�tique. Ces fausses perles s�int�grent dans les coiffures f�minines et s'associent � des perles de verre et d�agathe dans de nombreux colliers.

My translation (French speakers, please correct if needed!)
Amber:
Amber, limited to Mauritania, has prophylactic values. For a long time already, astute Syrian merchants traveled Saharan markets and major camps to exchange genuine amber balls of Mauritanian "meyyal" for well rounded and polished imitation plastic. These fake beads are integrated in the women's hairstyles and combine with glass beads and agate in many necklaces.

It makes sense that Syrian traders were working in the French Sahara countries since Syria was under French control from about the end of WWI to 1946, which includes the period of control by Vichy France (German-controlled).