The maximum amount that you can put into a trade is the available Cash for Trading. Basically, this is the total amount of money that you deposited in your BUX account minus the invested amount, minus the Ring Fenced amount (see also: what is the Ring Fenced amount?).
To explain this in more detail, we would like to give you an example:
Say, you deposited £1000. The Ring Fenced amount is then 5% of your total account value which is (5% of 1000) £50. If you haven’t invested anything yet, this means your available Cash for Trading is (1000-50) £950. If you now invest £600 you still have (1000-600-50) £350 left as available Cash for Trading.
A reminder: you can never lose more on a Trade than your invested amount (see: why can I never lose more than I invest?).
Take a look also at Why can’t I invest my whole account value (over £500) into one trade?
If you have any further questions in regards to this, please feel free to contact us via email at BUX Support.