Silver certificates were first introduced in the United States in 1878. The twenty dollar denomination used Stephen Decatur as the central portrait. The design was used for 1878 and 1880 before it was redesigned in 1886. 1878 $20 silver certificates are extremely rare. 1880 examples are not quite as rare, but they can still be valuable. When dealing with either series the major factor that determines the value is condition. Low grade silver certificates are occasionally available. However, notes in true very fine condition (or better) are very rare and can be very valuable.
If you have a $20 silver certificate then we would highly recommend that you contact us. We have paid tens of thousands of dollars for some of the rarer types. Send us an email with pictures of what you have and we would be happy to make an offer.