Sc: Neighbor Has Maintained Part Of My Property For Years—should We Document Permission?

Location: Upstate SC

I’m contemplating selling my house in the next year or two and a realtor I spoke to suggested getting property lines staked so I can ensure my fence is entirely on my property when I repair it and move it over if needed. Property lines were staked and it turns out my property line is much farther into my neighbor’s lawn than anyone anticipated. About 15’ at the roadside. They’ve mowed that area for as long as I’ve lived here and I think it’s possible the sprinkler system they once installed may be in that area as well. (They used it for 3 months before deciding the increase to the water bill was not worth it.) Do I need to get something in writing to document the understanding of the surveyed boundary and protect my ownership

Backstory: I moved in about 19 years ago and my neighbor (we’ll call him John) and I built up a friendly rapport and neighborly relationship. He told me about how the property lines aren’t perpendicular to the road and told me approximately where the line was between us. When I put up a fence for the dogs, most of it is among trees and used field fence. I approximated the lines based off my other neighbor’s fence and John was perfectly fine with where my fence stood. His granddaughter is now living in his home with some friends. We have no beef but we don’t have the same relationship as John and I had.

Could their long-term mowing and maintenance of this area give them grounds to claim ownership or rights to use it? Largely for aesthetic reasons I’m happy for the lawn to stay as it is, including them continuing to mow it, but I don’t want a future buyer to encounter a dispute when using the full property. Should I have something in writing acknowledging the surveyed boundary and documenting that their continued use is with my permission?