Say I spend 200 foreign currency on Jan 1.
300 foreign currency on April 1.

What exchange rate shall I use for my book-keeping? The exchange rate on Jan1 for 200 and April 1 for 300 ? Or use the year end Dec 31 exchange rate for all 500 ?

Also each back offers different exchange rate, say BOA and Citi might offer different rates. Which bank rate shall I use ?

Assuming this has to do with foreign exchange rates for tax purposes, then IRS lets you do it one of several ways:

1) you can use the AVERAGE annual exchange rate.

2) You can use a MDIAN exchange rate (tax rate at the wid-way point of the tax year, typically July 1st.

3) You can use the exchange rate on the date of the sale.

Whichever method you choose, YOU MUST BE CONSISTENT. If you use option #1 for the income parts of the return, you must use the SAME option for the tax parts of the return.