I often use scales::percent() and scales::dollar() inside inline RMarkdown code. I know this package was created for ggplot2 plots, but this use case is really handy.
As of now, using the later on negative values (i.e., scales::dollar(-200)) places the $ symbol before the negative symbol.
library(scales)
scales::dollar(-200)
#> [1] "$-200"
Created on 2019-07-15 by the reprex package (v0.3.0)