What is the cheapest broker to sell a large amount of US stocks say 6 figure USD worth of US stocks?

I am not sure where your US stocks are held in currently but it is likely in your current broker custodian account. If you do switch broker for the cheaper commission, you need to also transfer the shares over to a new broker. This incurs another cost for transferring shares.

For exchange rates on brokers, I cannot comment much, they all fluctuate differently.

For cheapest commissions, thinkorswim charges a flat $10.65 USD per trade of unlimited shares.

Standard chartered if you are a priority banking customer, charges 0.2% commission.