I don’t see the write off being enforceable now. If the owner of the encumbered land did not object at the time, it could be argued as an implied easement. Neighbor had the opportunity to prevent the situation of the septic being on his land or allow it. He apparently allowed it.
I see the op’s options as;
1. buy the land but live with the septic until it requires replacement.
2. Don’t buy the land.

