Why would they be toxic? Beans are grown for eating...I'm a bit puzzled by your question. You eat the immature ones along with the outsides, don't you. I think the remaining seed beans might be a bit tough, though.

my grandfather used to grow runner beans with white seed and he used to keep the seed, some for the following year and some he used to cook the same as you would butter beans, if that is any help to you

Runner beans seeds contain a hemagglutinin which is toxic, just a few being enough to cause stomach upset, and vomiting etc. I 've always eaten the young beans raw, it's one of the delights of early summer, but really, they should be cooked, to nullify the toxicity. Phil J

Well, we live and learn Phil! Pleased that you're still here to tell the tale. :-)))