Prussian state 25mm button by Eduard Sachs, Berlin.

Large 25mm size; The Maker: Eduard Sachs, Berlin
Eduard Sachs was a “Militäreffektenfabrik” (military effects/accessories factory) and held the prestigious title of “Hoflieferant des Kaisers” – supplier to the Imperial Court. This is a significant distinction, meaning the firm was officially appointed to supply the Kaiser himself.
Eduard Sachs Berlin appears on items dating back to around 1870/80 , and buttons bearing this maker’s mark are associated with the period 1889 to 1918 – spanning the reigns of Kaiser Wilhelm I through Wilhelm II, right up to the end of WWI and the collapse of the German Empire.
Embossed eagle with outstretched wings and a crown above its head, noted as possibly Prussian.
The Button Itself
large white metal (likely nickel silver or tombac) Prussian state or military official’s button. The design on the face is the Prussian eagle on a crowned shield – a classic heraldic device of the Kingdom of Prussia.  
The crown surmounting the shield on your button is the Prussian royal crown, firmly placing this in the Imperial German period (pre-1918).
Dating this is most likely pre-WWI, probably dating from the 1880s to around 1914. The quality of the white metal, the crisp detail, and the Sachs maker’s mark all point to a high-quality dress or full uniform button from the Kaiserreich period. The large size suggests it would have been a coat or greatcoat button rather than a cuff button.