I tell him his house is still there and we aren't selling his house yet and he keeps thinking he sold it to a friend, or wants to let the friend know what we are doing with the house. I keep telling him we are not selling yet, but he keeps asking me this.
Rather than telling dad that you are not selling the house "yet" can you tell him that you are having work done on it. (And that "work" can take as long as you need it to take)
For someone with dementia time is sort of meaningless so "yet" might not register with him. But having "work" done might have more meaning.
When you deal with others have them keep answers short and simple to lessen the confusion.
For some people if they intentionally confuse your dad I would just cut contact with them at least while dad is around. (Until they "get it" and comply with what you ask of them).
By the way I just read your profile and you indicate that dad is in Independent Living. This might be something that you want to look into changing.
Independent Living a resident can leave the building usually without informing anyone. There may come a time that it will be dangerous for him to be able to wander out. Also others in the building that are cognizant could possibly take advantage of him financially. (believe it or not there are some unscrupulous people living in communities and they can prey on people) Consider a change to Memory Care