Stocks can have negative values and buying them increases your cash

It doesn't happen everytime, but on my game i had VOD at -$1 and i bought 100 shares and my cash went up by 100, i gained 100 shares of VOD, and VOD went to $0.

You probably are just missing an abs() somewhere in your code.

Start by reproducing the negative-stock-price case in the game, then trace the stock purchase and cash-update logic. Confirm the behavior around VOD at -$1 and buying 100 shares. Done means stock values do not produce a cash increase when shares are bought, and the transaction leaves the stock at the expected value.

Written by the indexing model from the issue text.