A security official said Muhammad Ismail, 50, was killed by a bullet in the chest from the Syrian side of the border on Tuesday.
But another source said the shot came from a position maintained by a pro-Syrian Palestinian group, Fatah Uprising.
A senior official in the group denied any involvement.
Sources said Ismail was among a team surveying the border.
Prime Minister Fouad Siniora has said the government is planning to revive an agreement between Lebanon and Syria on the border's demarcation, but gave no further details.
The issue of armed pro-Syrian Palestinian groups in Lebanon has moved to centre stage after the UN Security Council resolution last year demanded the withdrawal of Syrian troops from Lebanon and the disarming of all armed groups.
Syria ended its 29-year military presence in Lebanon in April.