Keep in mind, that any of the 'tax reasons' wouldn't be due until mid Sept..
I have no idea why you say mid Sept is tax due date. These are restricted stocks. Unless they chose section 83(b) election at grant - they would have paid tax at grant which I highly doubt, the company is supposed to either withhold tax in the first paycheck immediately after the time of vest (potential problem is tax liability exceeds one paycheck, thus employees are supposed to be educated before vest date) or sell shares on open market and sale proceeds forwarded to company to cover taxes due to vest of restricted stock. Since these restricted stocks vest upon Lovenox approval, so they have to do it within very short period of vesting. This is different from stock option where you don't have tax liability at vest rather only at excercise.