you sold stock and got $30K. on your W-2 in some year there was income added for the RSU that gives you a cost basis in the stock. since there is a cost basis the full $30K is not taxable. what happened is the brokerage, as required by law, submitted to the IRS a 1099B showing the $30K as the sales price but with no cost reported - result the IRS thinks you have a $30K gain. you should have gotten a 1099B from MS to match the one they sent to the IRS. it may have a cost basis on it. it may be that under the goofy tax laws we have, they are not required to submit the cost basis to the IRS even if they have it.
For RSUs, the cost basis should be the fair market value (FMV) of the shares on the day they vest.
