Don't change available amount when selecting Trade amount

Currently, when buying a stock, the amount available to invest is shown next to the input for how much of the stock I want to buy.

However, that available keeps changing, depending on the amount I choose to trade, which is confusing. I only expect it to change once the trade has been executed, and should be static on that screen. The mental math to calculate how much would remain in the account after the trade is quite easy and the app shouldn’t be doing it for people - it should just be displaying the cash available.