Just to turn the thing on its head for a minute: how important is that 2.5 acres to your 100+ acre parcel? If the other family has lived for years (generations?) thinking of it as theirs, it may have more value (economic, emotional) to them than it does to you.
If it were me, I would first try to gain a better understanding (from the legal record, from the title company, maybe from an attorney, and finally from the neighbor), as to precisely why/how they thought the 2.5 acres was theirs.
I would then have whoever is best at explaining things very clearly sit down with them, and take them politely through the docs to show why their understanding was incorrect.
Depending on how that goes, and whether they remain civil, and how much you like/need that 2.5 acres, I might tell them I'd let them have it for a proportional share of the purchase price.