«بدرة عين»
A purse containing a large sum of money.
«قطرة ماء»
A drop of water.
The purse of ten thousand dirhams represents abundance and the drop conveys minuteness. Despite the shared designation nawāl, an exaggerated difference in magnitude and worth appears.
The rhetorical steps are (1) a common category of giving, and (2) a marked contrast within that category. Merely assigning unrelated names to two unrelated objects would not produce this particular figure.