"Rounding error" is a specific term so I can't think of any suitable synonyms. It's usually only used when you are literally talking about rounding or decimals in math or science.
However, with the sentence you gave, it looks like you want something else that means $10,000 is nothing to this very rich person. There are a couple of colloquial expressions I can think of that might be good in this sentence:
10 thousand dollars, for him, is just chump change. <- if you are talking about him spending $10,000 like it's nothing
10 thousand dollars, for him, is just a drop in the bucket. <- if you are talking about $10,000 being only a tiny part of his massive fortune.
I would not use "oddment" though. I am a native English speaker and have never once heard anyone say that in any context, so it will sound weird if you use that.