PLEASE HELP IF POSSIBLE --- FIFO MODEL ARCHITECTURE FOR FOREIGN EXCHANGE

There are several FIFO products/cost of stock models online but they do not work in my application. I am required to calculate FX capital gains derived from USD balances on a first in first out (FIFO) basis. It's for capital gains tax using a cross of US dollars and Euro currency. The base currency is Euro, where today's rate eg. is 1.1109 USD for 1 Euro. I need to be able to track flows in and out of account the inbound pegging to the prevailing rate of that day and the outbound or (Dr), crystallizing a capital gain or loss based on the use of funds from a FIFO model. This is an ongoing track.

I would really appreciate any assistance on the architecture - I am really stuck. I attach the type of structure I have been grappling with.

Any help would be amazing.  Thanks