Broker Cash, Strange Metric

### Expected Behavior
Start with $1000
Buy $100 of stock
self._broker._cash should be $900.
Sell $150 of stock
self._broker._cash should be $1050

### Actual Behavior
Start with $1000
Buy $100 of stock
self._broker._cash doesn't change
Sell $150 of stock
self._broker._cash is now $1050 (cash += closed_trade.pl)

I don't understand this behavior. The variable is named cash, but it's not really the current cash value.

If you want to make trades based on your actual available cash, self._broker.margin_available seems to be what you want... but that seems strange.

Am I missing something here?

Start by tracing the broker's _cash and margin_available values through the buy and sell examples in the issue. Determine whether the observed behavior is intentional or a defect, then define the expected cash semantics and verify the behavior with focused trading scenarios. The issue does not name a source file or test.

Written by the indexing model from the issue text.