EXCLUSIVE

The man whose truck was carjacked by Christopher Dorner during his cop killing rampage -- and who subsequently notified police -- doesn't deserve a cent of the reward money ... so says Riverside County, California.

TMZ broke the story ... Richard Heltebrake sued a bunch of government bodies in SoCal that promised a million-dollar reward for information leading to the capture of Dorner, who murdered three police officers in February, 2013.

Heltebrake claims his tip was crucial in tracking down Dorner -- and he demanded the full reward, which was collectively financed by several municipalities, including Riverside County, the City of L.A., and the City of Irvine.

But now, Riverside is saying Heltebrake deserves jack squat -- because his "tip" regarding Dorner's whereabouts came in AFTER police already knew where Dorner was.

Lawyers for Riverside cite Heltebrake's own account of his call to police -- where Heltebrake describes hearing a "serious volley of gunshots" before calling a local deputy. Riverside claims those gunshots were from a firefight between Dorner and police.

Translation: his tip came in too late.