The US issued both $20 coins and $20 bills in 1928. Please see the appropriate Related Question.
1928
The green seal indicates that your bill is a Federal Reserve Note. All 1928 B $20 bills were printed as FRNs. Please see the question "What is the value of a 1928 B US 20 dollar bill?" for more information.
A 1928 US Peace Silver Dollar is worth approximately $20 to $130 if it is a coin that has been circulated. Uncirculated coins can command upwards of $600.
Please check your bill again and post a new, separate question. There were no series letters on 1928 $20 gold certificates.
President Andrew Jackson is on the face of the twenty-dollar bill. President Andrew Jackson first appeared on the twenty-dollar bill in 1928.
The Denver mint did not strike any silver dollars in 1928. You may have a 1928-S where the S is indistinct. This coin is worth $20 to $30 depending on condition.
If you mean a 1928 Peace dollar with No mint mark It's the key date to the series the value in VF-20 is $475.00. I suggest you take it to a coin dealer, because I have seen many 1928 S mint coins with the mint mark removed.
If you mean a 1928 Peace dollar with No mint mark It's the key date to the series the value in VF-20 is $475.00. I suggest you take it to a coin dealer, because I have seen many 1928 S mint coins with the mint mark removed.
Please check your bill again. The last $20 silver certificates were printed in 1891. If it has a green seal, then as indicated by the banner across the top of the portrait the bill is a Federal Reserve Note. Please see the question "What is the value of a 1928 US 20 dollar bill?" for more information.
There were no 1928 A $1 United States Notes printed, only 1928 "plain". If that's what you have it's worth $20 to $50 retail, depending on how worn it is.
If circulated, retail prices range from $10 to $20 depending on condition.
All 1928 $20 Federal Reserve Notes carried the phrase "Redeemable in Gold", although they weren't gold certificates. There were also gold certificates with that denomination but they have gold seals and lack the words Federal Reserve Note. Please see the question "What is the value of a 1928 US 20 dollar Federal Reserve Note?" for more information.