Whenever you see ${variableName} in the request, it means the variable hasn't been defined. Most likely, it's an issue with how XPath is setup. I've never used that component, so unfortunately I can't help you there.
However, I've found the post processor Regular Expression Extractor to be very easy and accurate.
The structure would like this:
1st Soap Request
-- regular expression stored to "var"
2nd Soap Request, using ${var}
BlackGaff
2010-09-07 14:43:31