I posted a job on Elance to see if I can solve the problem posted in Webmasters (here).
In the webmasters question or comments, can I link to the Elance job?
I'd say no, don't link to an Elance job from a question on any Stack Exchange site. You shouldn't even need to link to another Q&A site, bulletin board, or forum for supporting information (unless your question is specifically about one of those things). A question you post here should be complete and self-contained.
As for posting job links in comments, I'd have say I'm against that too. Stack Overflow isn't a job board, and if we allow people to post a question with an Elance link in the comments, then the question will just be an excuse to post a job ad. This site is for programming questions between programmers, not employers looking for workers. If a link isn't essential to the question, just leave it out.
Offers of payment for questions are generally frowned upon on SE, for various reasons. I wouldn't put it directly into the question - it could be reason for some to downvote, closevote, or edit it out.
However, I see absolutely nothing wrong with posting an elance link in a comment.