I needed something that would allow a client to provide name, email, invoice number, and amount. All of the solutions I could find were either for the older version of Webform, or they were too cumbersome... too many modules required... not enough simplicity and control. Many of the solutions would only allow for "donations" of fixed amounts.
My solution was to adapt the solution found here (which was written for a specific scenario, also using the older version of Webform), adapt it for my needs, and recode for Webform v4.
Note that this solution requires that you enable the "PHP Filter" module, which is a potential security issue if configured incorrectly.