ZBW Controller described a loss of separation event during a parallel vectoring descent attempt; the reporter identified a clearance readback error that was not immediately recognized causing the separation loss.

Narrative

I was working the RADAR at ALB Sector. There was Air Carrier Y going to EWR at 160 and Aircraft X going to CDW at FL240. I wanted to get Aircraft X below Air Carrier Y so I descended him to 140. I cleared him direct AGNEZ and saw that was enough so I turned 10 degrees; then another 10 degrees to parallel the Air Carrier Y flight. I asked him to maintain 1;500 FT for greater on his descent and he said he was doing 2;000 FT. I then proceeded to tell him leaving 150 thousand cleared direct AGNEZ. He read back 150 direct AGNEZ. I thought he read back the right clearance and in reviewing the tapes he thought he was cleared to 150 and turned towards AGNEZ causing lost of separation. When I asked him to be below 150 in 1 minute he said he was leveling at 150 and I inquired if he was descending to 140 then he proceeded to tell me he was descending to 140. His phraseology was not clear in the situation. I should have verified with the aircraft about the read back and trusted my instinct when I thought he had turned. I should have also had him report out of 150.