ZTL CTR REVERSED DSCNT CLRNC TO A CARJ1 FROM FL270 TO FL240 BECAUSE OF OPPOSITE DIRECTION TFC AT FL260. SEPARATION WAS LOST.

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CRUISING AT FL270 ABOUT 30 NM NW SPA; I WENT TO #2 RADIO TO CONTACT OPS REGARDING BOY WHO THREW UP IN CABIN. HEARD ATC (ZTL) CLR US TO DSND TO FL240; PF ACKNOWLEDGED; I CAME ON HOT MIKE TO CONFIRM THE NEW ALT SET IN THE ALERTER; AND PF INITIATED DSCNT; SELECTING VERT SPD 1500 FPM. I WENT BACK TO CONVERSATION WITH DISPATCHER AND SOME SECONDS LATER; OVERHEARD ATC ASK MEEKLY; IF WE COULD STAY AT FL270. PF ASKED ATC TO REPEAT; I TOLD DISPATCH TO STAND BY; CAME ON HOT MIKE AND SAID 'HE WANTS US TO STAY AT FL270.' THEN CHKING TCASII AND SEEING FL260 TFC AT 10 O'CLOCK POS AND APPROX 8 MI CONVERGING; I SPUN THE VERT SPD WHEEL UP TO STOP DSCNT-START CLBING WHILE ATC REPEATED HIS QUESTION. FO REPLIED YES; WE CAN CLB BACK TO FL270. ATC QUERIED US AGAIN; AND FO SAID THAT WE WERE CLBING BACK UP TO FL270 AT THIS TIME. I SPOTTED TCASII TFC VISUALLY AND RPTED THAT TO ATC. LOWEST ALT REACHED WAS APPROX FL267. AS THE FL260 TFC CAME INSIDE 5-MI RANGE ON TCASII; WE WERE CLBING BACK THROUGH FL268. WE WERE LEVELING AT FL270 AS THE TCASII TFC PASSED ABOUT 2 MI BEHIND AND TO THE L. DURING THE ENCOUNTER; WE GOT A TA ON THE TFC; NO RA; AND NEVER SAW THE TCASII TFC CHANGE ALT ON TCASII. A FEW MINS LATER; CTLR SAID 'OK; LET'S TRY THIS AGAIN: 'ACR X; DSND AND MAINTAIN FL240.' FLT CONTINUED WITHOUT FURTHER INCIDENT. I'M VERY THANKFUL THE CTLR CAUGHT HIS ERROR BEFORE WE HAD DSNDED ANY FURTHER. THERE SIMPLY WAS NO AVOIDING A LOSS OF SEPARATION AFTER RECEIVING THE DSCNT CLRNC; GIVEN THE TIME THAT ELAPSED UNTIL THE CTLR CALLED US BACK. I'M ALSO GLAD THAT I'M IN THE HABIT OF TRYING TO MONITOR ATC AT ALL TIMES; EVEN WHEN USING THE OTHER RADIO GETTING ATIS; CALLING DISPATCH; COMPANY OPS; ETC.