What di you mean by signature? Leave all exif intact of course! I assume as your first sale is to America, that this is a digital sale. What rightsdo they have to the image?
If they hve seen a preview, and the signature is a small black name in the bottom corner, I would be tempted to leave it.
It depends a lot on the image, and how well the signature blends.
Obviously, once they have paid a visible watermark is a no-no (I did a bit of playing with de-moire, but it is difficult)
If however, it was a physical print, I might be tempted to remove the signature from the front, but back-print full name and address and copyright details.