from Hooyou.com

http://www.hooyou.com/lc/index.html
In Labor Certification, the U.S. sponsoring employer is the petitioner and the prospective foreign employee is the beneficiary. Since the Labor Certification is filed for a prospective employment, it does not matter whether the alien works for the sponsoring employer during the application or after it is approved. Meanwhile, the sponsoring employer does not necessary to keep the alien’s employment. However, the foreign employee has to work for a reasonable period of time for the sponsoring employer after s/he received permanent residency.