DEPARTING SMT CLIMBED INTO CONFLICT WITH INBOUND SMT RESULTING IN LESS THAN STANDARD SEPARATION.

Narrative

I WAS WORKING AS AN FPL SEC 19 AND 20; COMBINED; A LOW ALT SECTOR FL230 AND BELOW; WHICH COVERS A LARGE PART OF NEBRASKA AND KANSAS AND SOME OF COLORADO. SMT X WAS WBOUND ON V6 45 MILES EAST OF LBF AT 6000' INBOUND TO LBF. AT THAT TIME SMT Z REQUESTED A CLRNC FROM LBF TO FAR AT FL190. SINCE THE WBOUND ARRIVAL TO LBF WAS STILL 45 MILES OUT; I CLRED SMT Z AS REQUESTED. I LATER CLRED SMT X TO DESCEND TO 5000' AND REPORT THE ARPT IN SIGHT AND ADVISED I WAS PLANNING A VISUAL APCH. I THEN CALLED LBF FSS WHO WAS WORKING SMT Z AND INQUIRED WHEN HE WOULD BE OFF. I WAS TOLD ABOUT A MINUTE. WHEN SMT Z DID GET OFF; I ASSUMED HE WAS TAKING OFF NORTHWBOUND AWAY FROM MY INBOUND TFC. I ASSUMED WRONG! SMT Z TOOK OFF SEBOUND INTO MY TFC WHICH WAS CORRECT CONSIDERING THE WINDS. WHEN I REALIZED I MIGHT HAVE AN ERROR I ASKED THE DEPARTING SMT Z WHOM I NOW WAS WORKING TO MAINTAIN 4000'. SMT Z ROGERED BUT COULD NOT STOP AT 4000'; HAVING BEEN SO CLOSE TO THE ALT WHEN I GAVE THE CLRNC. I FEEL THE ERROR WAS COMPLETLY MY FAULT. I DID HAVE SOME DISTRACTIONS AT THE TIME; I WAS ON THE SECTOR FOR 2 HRS ALONE; HOWEVER WITH MY VAST EXPERIENCE (30 YEARS) I SHOULD HAVE KNOWN WHICH DIRECTION THE ACFT WAS TAKING OFF. THE ERROR WAS CLASSIFIED AS MINOR (4 MILES AND 500')