Mistake of Fact?

Employer properly deducted and remitted participant deferrals.  But due to some accounting mishap on their end, they won't up contributing additional amounts for several participants.  It wasn't a correction or anything.  All the amounts were properly withheld and remitted the first time.

So say Jimmy had a $100 deferral from his paycheck and the company deposited that timely.  But when they went to do in-house balancing, it looked like he was short $15 so they sent that in for him, too. (the $15 was NOT withheld from pay)

What's the correction?  Put the funds in suspense account?  Should it be sent back to the company (with earns, if any) as a MOF?

I am leaning toward the latter.

In your hypo, the $100 was the correct deferral, was actually withheld, and was fully deposited on a timely basis, yes? Then some accounting report seemed to incorrectly indicate there was a $15 deposit shortfall which was then unnecessarily made up via another deposit? 

I think either method for correction would be acceptable.

I'm guessing it depends on if the ER took a deduction  for it.  If they took $115, then put it in suspense.  If not MOF.