Master is used in many contexts in English that have nothing to do with slavery. The first definition given by the OED is “a person or thing having control or authority”.
An arbitrarily chosen quotation: “ M. le Comte Benedetti, French Ambassador at Berlin..sped to Vienna with the latest proposal of his master.”
Certainly, this ambassador Count Benedetti was not a slave.
An arbitrarily chosen quotation: “ M. le Comte Benedetti, French Ambassador at Berlin..sped to Vienna with the latest proposal of his master.”
Certainly, this ambassador Count Benedetti was not a slave.