Can employer limit 403b contributions to employees employed on date of contribution?

Employer wants to limit contributions to situations only if employee is employed on date of contribution.  Currently, employee vests in a contribution and then the contribution is made let's say 1 month after the vesting date.  Employer wants to amend plan so that if the employee is not employed on date of contribution, then they don't get the contribution.  Is that even permissible ?

First question is the plan's allocation formula and requirements to share as indicated in the plan document.

I am not quite sure what vesting has to do with this, since vesting accrues when an employee has 1000 hours in a plan year.  And I would assume that you are talking about a NonElective employer contribution and not matching.  The plan could use "last day" as a requirement and only be obligated to contribute for employees employed on the last day of the plan year.  I do not think the requirement the employer seems to be designing could be done in a pre-approved plan document.

@Patricia. Thank you.  Yes, this is a pre-approved plan, and yes currently the allocation is 1000 hours of service in a plan year for the employer non-elective.  So yes, I agree I believe they can do "last day of the plan year" but not what they're trying to do -- which is a requirement that the e/e be employed on date of contribution.