Coercion, bribe or ... election promises?

A big problem, in any election (electronic or off line), is the problem of coerced or bought-out votes. In general, no elections system or electoral authority can completely eliminate voter-coercion. If a voter chooses to sell his vote and actually carry out his end of the deal, then it must be considered part of the due process of election and his vote must be honored. If on the other hand, the agent of coercion is unable to convince the voter , then a forced vote is anomalous in the process.

Although not evident at first, the line between a "valid and freely chosen vote" and "invalid and coerced vote" is very thin. Finally, how does one different between coercion and campaigning? Try the following two statements:

"If you vote for me, I will pay you $100."
"If you vote for me, I will cut your taxes by $100."

Are they different? If so, how?