Advise the client that you should amend the 1099 and that they should notify their vendor of the $1,000 shortage. If the client says no, then document it and move on. More than likely that $1,000 is not going to cause any issues on the vendor side of things as their revenue is probably greater than the 1099 anyways. You'll have to evaluate whether to keep the relationship or not if they say no. Where do you draw the line if they say no in the future. Probably overkill with this, but just a thought.