(plaintiff, estate executor, filed claim for refund of estate tax and IRS moved for summary judgment; under I.R.C. Sec. 6511(b)(2)(A), amount of refund limited to portion of tax paid within look-back period immediately preceding filing of claim (3 years plus the period of time for filing return); executor got extension for filing return and then got second extension; court held that IRS had no authority to grant second six-month extension; look-back period did not cover period during which payment made so executor not able to file refund claim; equitable estoppel not available to extend timeframe for extending time to file refund claim; IRS motion granted).