Short story about office mate's exotic vacations where the colleagues fake a Martian holiday and he goes on it for real

I read this story before 1985, and it had a kind of 1960s-1970s setting. A very ordinary office worker would take a vacation every few months to exotic locales like Tahiti or Tibet, showing off pictures to his co-workers, who can’t believe he could afford this on a salary comparable to theirs; when asked about this, he vaguely replies about finding special discounts which they can never find themselves. Some suspect that he’s faking these trips, though the pictures look extremely real.

Annoyed, they finally rig up a faux "Martian vacation" which one of them takes, with pictures of obviously fake rockets, canal scenes, and cardboard cut-out "Martians" posing with tourists to try to "one-up" him. He takes this all seriously, however, showing surprise that he’s never heard of this tour opportunity before; they vaguely explain that some (non-existent) tourist agency on some hard-to-find street handles it. A month later he disappears for a bit, then returns saying "I found that agency!" and thanks them for the tip as he shows them pictures of his real Martian vacation, posing by actual aliens and canals, etc., leaving everyone dumbstruck.