My understanding is that there is an increased risk of cancer in retained testicles.
Can the vet feel the retained one at all,sometimes you can,othertimes they are well up inside the dog,and can be difficult for the vet to locate.
Borders who are neautered tend to loose their coats ,often go all soft and fluffy - some can't be stripped afterwards.Don't know who important that is to you.
I'd think about just having the retained one removed.
I personally would't want to breed from a dog with a retained testicle,just in case.