前边 走来 一 个 外国人 and 前边 来 一 个 外国人. mean same, in the first sentence the important matter is that he walkes (not runs or rides) in the second the important matter is that he's approaching towards us 前边 走 一 个 外国人 - means he walks in front of us, probably crossing our way or alongside and it would be 走着, 走了，来着 or 来了 anywhere but "Elementary Very Easy Chinese level 1 with Fun" textbook

Ibet the first one is acceptable while the latter two are odd and unusual. 前边走来一个外国人—— There is a foreinger at front coming to here.