The effect of an employer withdrawal of an I-140: Ron Gotcher

http://imminfo.com/News/Newsletter/2010-07/employer-withdrawal-of-I-140.html
The effect of an employer withdrawal of an I-140 depends on the employee's processing stage at the time of the withdrawal. There are three significant "break points" for purposes of this discussion:
 
First, the employer withdraws the I-140 before it is approved.
Second, the employer withdraws the I-140 after it has been approved.
Third, the employer withdraws the I-140 more than 180 days after the employee has filed for adjustment of status.

Let's look at these situations individually.
If an employer withdraws an I-140 before it is approved (unless in the context of a concurrent I-485 filing - discussed separately), then the employee gains nothing from the filing. The priority date does not become vested until the I-140 is approved. In this situation, it is as though the PERM and I-140 had never been filed.
 
If an employer withdraws an I-140 after it has been approved, however, the employee is entitled to keep the priority date and use it for any subsequently filed I-140. It doesn't matter that the new I-140 may be for a different job, a different employer, a different location, or even a different preference classification. Once an I-140 is approved, the employee "owns" the priority date forever.

If the employer withdraws an approved or still pending I-140 more than 180 days after the employee filed for adjustment of status using that I-140, the withdrawal is of no consequence. The CIS must regard it as still valid for adjustment of status purposes. If it has not yet been approved, then the CIS must look to see if it was "approvable" at the time it was filed. If so, then the I-140 will remain valid.

If the employee leaves the sponsoring employer, and the employer withdraws the approved I-140, then after the withdrawal the employee may no longer use the approved I140 as a basis for an exemption from the six year H1B limit. If the employee's new employer obtains a new H petition before the old employer withdraws the petition, the employee will be given H status for up to three years (depending on the petitioner's ability to justify the length of time requested).