By Tom Miles GENEVA, Jan 17 (Reuters) - Agreement may be near on a global treaty to reduce the use of toxic mercury, but arguments about a few tens of millions of dollars are taking the talks to the brink, a U.N. official involved in the Geneva negotiations said on Thursday. Another official predicted that the talks were likely to bring a deal late on Friday or in the early hours of Saturday, but it was still unclear whether the eventual text would be "a Swiss cheese" - full of holes.