Basically they are using your company to support their visa application, so unless you want to be dragged into a case for illegal immigration, do NOT send the letter.
Anyone with a reasonable amount of Gold to sell can get a visa or ship it.
Some of the main Gold scams are: 1) The most common is sellers asking you to pay upfront costs for shipping of your Gold, Shipping is expensive and this can be many 1000's of dollars.
These proofs are then used to run scams in your name, especially passports!These are often backed up by equally corrupt lawyers or advocates in Africa.A variation on this theme is the girlfriend or boyfriend befriending you and then promising to come visit / marry and just needs a few thousand dollars to release gold to sell to cover their trip. There are many variations on the scams listed and of course every one will claim that they are the real thing and have had problems with fake buyers before and dont want to get caught again.It doesn't really matter what is being asked for, if you pay up front don't expect to hear from the scammer again.A thousand dollars may not be much for you but most Gold comes from countries where a 00 goes a long way.