Severance plans vary widely, and whether or not you get severance depends on the terms of the plan. In your case it seems as though the terms of the plan require you to work until the closing in order to qualify for severance. Your employer can impose that term on the plan. If the closing has been delayed, and you resign to take another position before the closing date, you will probably be considered to have voluntarily quit and therefore not eligible for severance under the plan.
Answered on Jul 30th, 2012 at 1:00 PM