Crypto and FX positions have a cost-basis problem. A user buys 0.1 BTC at ₦60M/BTC, 0.1 BTC at ₦70M/BTC, and 0.1 BTC at ₦80M/BTC. Now they sell 0.15 BTC. Which lots did they sell? Tax outcomes depend entirely on the COST BASIS METHOD: FIFO (sell the oldest lots first) reports lower gain in a rising market; LIFO (sell the newest first) reports higher gain; HIFO (sell highest-cost first) minimizes taxable gain. Many crypto-fiat fintechs let users PICK the method (with tax-form generation reflecting the choice); regulators in some jurisdictions force FIFO. The ledger needs to track lots per user per asset: opening lot, lots consumed by each sale, remaining lot balance. This lesson posts a sale under FIFO and shows the per-lot consumption logic.