Sale of Re-platted property - Tax question

[Doug Block]

Looking for some advice on how I should treat this situation when I file my taxes this year.

I bought a piece of property a few years ago and replatted it and split it into 2 (not equal amounts). Lot A, the main lot, was larger and has a house on it. Lot B, was smaller and no house. So when we split, the mortgage company re-appraised Lot A and determined it was well above the outstanding loan amount, so they released Lot B from the lien. Essentially meaning, I owned this free and clear.

I then sold Lot B.

So my question is how do I deal with the gain on the sale of Lot B. Is my acquisition price $0? Or do I allocate a certain portion of the original purchase price between Lot A and Lot B? And if so, what should I use as the measurement?

TIA for any feedback or advice.