Is it permissible (halal) or forbidden (haram) to subscribe to shares of companies that have not yet been established, and before the shares are listed on the market?

It is permissible to subscribe to permissible companies during the founding stage, but it is not permissible to sell their shares for more than their real value until they acquire assets and commence their permissible activities. If the share is sold before that, it must be sold at the same purchase price, as this falls under the category of currency exchange (sarf), which requires equivalence and immediate possession if the two exchanged items are of the same kind.