If by any chance he comes this way, I'll let you know right away.
Literal
By-chance he [subject-が] this-way [to-へ] came-[conditional-たら] immediately will-inform.
万が一 and 万一 mean the same thing — 'one in ten thousand, if by the slightest chance.' Inserting the particle が between the numerals lends 万が一 a slightly more emphatic, measured feel; both are common in formal contingencies and warnings. Combined with ~たら, it sets up an unlikely-but-prepared-for hypothetical.