MLG ACFT CLRED TO CROSS INTXN AT FL280. CTLR INTERVENED WHEN A CONFLICT OCCURRED AND THOUGHT FLC HAD BEEN CLRED TO FL260. AN IMMEDIATE DSCNT CLRNC WAS GIVEN.

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APPROX 40 NM SE OF CGI VOR; WE RECEIVED A CLRNC FROM ZKC TO CROSS CAPE GIRARDEAU VOR (CGI) AT FL280. FO READ BACK CLRNC AND SET FL280 IN ALT ALERT. I VERBALLY RECONFIRMED THE XING ALT OF FL280 WITH THE FO. APPROX 7 MI (DME MILEAGE) SE OF CGI AS WE WERE LEVELING AT FL280; ZKC ASKED IF WE WERE GOING TO MAKE THE FL260 XING. FO REPLIED THAT WE WERE CLRED TO FL280. ZKC REPLIED WITH 'DSND IMMEDIATELY TO FL260; CROSS CGI AT FL260 RPT OUT OF FL270.' I STARTED AN IMMEDIATE DSCNT BY DISCONNECTING THE AUTOPLT AND PUSHING THE NOSE OVER AS RAPIDLY AS POSSIBLE WITHOUT CAUSING NEGATIVE G'S AS THE SEAT BELT SIGN WAS NOT YET ON. DURING THIS DSCNT I GLANCED AT THE TCASII AND OBSERVED A TARGET AT APPROX 2 O'CLOCK AND 7-8 NM AT 600+ FT. WE DID NOT SEE ANOTHER ACFT NOR DID THE TCASII ANNUNCIATE ANY KIND OF A WARNING OR ADVISORY. IN REFLECTING ON THIS EVENT IT IS DIFFICULT TO ASSESS BLAME FOR WHAT MAY HAVE BEEN A POTENTIAL CONFLICT. I AM 99% CERTAIN THAT BOTH THE FO AND MYSELF HEARD THE CORRECT CLRNC (FL280). THE 1% DOUBT IS CAUSED BY THE FACT THAT THE STANDARD CLRNC ON THE CAPE GIRARDEAU TRANSITION IS TO CROSS CGI AT FL280; AND THEREFORE COULD IT BE POSSIBLE FOR BOTH OF US TO HEAR ONE THING AND SET ANOTHER? THIS WOULD BE VERY UNLIKELY AS THE READBACK TO ATC; PLUS THE RECONFIRMING BY THE PF WOULD MAKE THIS ALMOST IMPOSSIBLE.