Ah OK, I see. There are two intertwining issues here, and the confusion came from the fact that they both cause the same error message. The two issues that you need to look out for are the following:
The reason that the $ signs are problematic is that the codegen launches a subshell to invoke ffmpeg. This subshell will interpret the $ signs as special shell metacharacters, so the whole thing gets expanded to a filename that doesn't exist. It might be possible to do some shell quoting magic to get such filenames passed correctly to ffmpeg, but even if it works it's going to be ugly. This is a bug in the codegen. The workaround in the meantime is to rename the files.
The second thing to watch out for has nothing to do with $ signs. But rather, the filename in the "-s" input file list is relative to the current directory, so make sure that the files referenced in the list actually reside there, or that you are instead referring to these files by their absolute path name.
Andrew