ARTCC RADAR CTLR DSNDED A COMMUTER CL65 TO FL210; BUT THE PLT READ BACK FL200 INSTEAD; RESULTING IN LTSS WITH A BE60 AT FL200.

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I WAS WORKING PKB RADAR POS WITH LIGHT TFC AND LIGHT COMPLEXITY. DUE TO MODERATE CHOP AT FL240 AND ABOVE; SEVERAL ACFT WERE REQUESTING LOWER ALTS TO GET OUT OF THE CHOP. NGA Y; A BE60; WAS ENRTE FROM RDU TO CMH AT FL200. I OBSERVED ACR X BEING RADAR HANDED OFF TO ME FROM ZDC. AS I STARTED TO ACCEPT RADAR ON ACR X; ZDC TOOK BACK THE HDOF AND CLBED HIM TO FL240 PER PLT REQUEST. APPROX 8 MINS LATER; ACR X STARTED EXPERIENCING CHOP AT FL240 AND REQUESTED LOWER FROM OUR 'HIGH' SIDE RADAR. I ACCEPTED RADAR ACR X APPROX 45 MI E OF CRW. WHEN ACR X MADE INITIAL RADIO CONTACT WITH ME; I ACKNOWLEDGED FOR ACR X TO DSND TO FL210 DUE TO TFC NBOUND (NGA Y) AT FL200. AS ACR X STARTED TO READ BACK THE CLRNC; SEVERAL SITS OCCURRED SIMULTANEOUSLY. ACR Z CALLED DSNDING TO FL240 AND STATED HIS RESTR COULD NOT BE MET DUE TO REDUCED AIRSPD DUE TO THE CHOP. I WAS ATTEMPTING TO COORDINATE WITH ALL SECTORS INVOLVED TO PROVIDE INFO ABOUT ACR Z NOT MAKING HIS RESTR. AT THE SAME TIME ANOTHER ACR RPTED PKB ARPT INSIGHT; AND I CLRED HIM FOR VISUAL APCH TO RWY 21. THEN I COORDINATED HIS POS TO PKB TWR. ALSO; A C130 TRAINING FLT WAS HOLDING AT OM FOR PKB AND REQUESTED RADAR VECTORS FOR VOR 21 APCH TO PKB. I THEN WENT BACK TO ACR Z AND RECLRED HIM TO FL210 AS PER LOA WITH ZOB. AS ACR Z AND THE C130 WERE ACKNOWLEDGING THEIR CLRNCS (I WAS WORKING 3 SEPARATE FREQS); ACR X READ BACK DSCNT TO FL200. I DID NOT COPY THE MISSED ALT INFO DUE TO BLOCKED XMISSIONS FROM OTHER FREQS; SO I ASSUMED THE CORRECT ALT WAS RECEIVED. I WAS MADE AWARE OF THE PROB WHEN CONFLICT ALERT ACTIVATED AND I OBSERVED ACR X MODE C READ BELOW FL210 WHICH WAS IN THE DATA BLOCK. I ADVISED ACR X THAT HIS ASSIGNED ALT WAS FL210; AND HE RESPONDED 'OUT OF FL200 FOR FL210.'