One advice generally given to prevent yichud is to give a key to a neighbor to make a pesach pasuach.
Does it work if you just tell the neighbor, “drop in sometime tonight” ?
I would think that you should need to say ” Come in as much as you want” or something similar to that because if you don’t , once the neighbor leaves it should be yichud because they know the neighbor is probably not coming back!
What you are saying is correct, he should say drop in as often as you want. Otherwise after he already came once, there is no reason to fear that he might come a second time.
As a side point the heter of giving a key to a neighbor who will come in, is in order to make the yichud have a shomer.
Gmar chasima tova