"common form" of a derivative). The Islamic finance equivalent of a conventional call option (where the buyer has the right but not the obligation to buy in the future at a preset price, and so will make a profit if the price of the underlying asset rises above the preset price) are known as an urbun (down-payment) sale where the buyer has the right to cancel the sale by forfeiting her down-payment. The Islamic equivalent of the "premium" in a conventional call option is known as a "down-payment", and the equivalent of the "strike price" is called the "preset price". A put option (i.e. where the seller has the right but not the obligation to sell at a preset price by some point in the future, and so will profit if the price of the underlying asset falls) is called a 'reverse urbun` in Islamic finance. Short-selling (though not technically a derivative) is also forbidden by conservative scholars because the investor is selling an item for which he never became the owner. However "some Shariah-compliant hedge funds have created an Islamic-short sale that is Shariah-certified". Some critics (like Feisal Khan and El-Gamal) complain it uses a work-around (requiring a "down-payment" towards the shorted stock) that is no different than "margin" regulations for short-selling used in at least one major country (the US), but entails "substantially higher fees" than conventional funds. Wa'dWa'd (literally "promise"), is a principle that has come to underpin or to structure shariah-compliant hedging instruments or derivatives. Conventional hedging products such as forward currency contracts and currency swaps are prohibited in Islamic Finance.Irfan, Heaven's Bankers, 2015: p. 144 Wa'd has been called "controversial" or a mimicry of conventional products and "'Islamic' in form alone". A "Double Wa'd" is a derivative that allows an investor to invest in and receive a return linked to some benchmark, sometimes ones that would normally be against shariah—such as an index of interest-bearing US corporate bonds. The investor's cash goes to a "special purpose entity" and they receive a certificate to execute the derivative. It involves a promise that on an agreed day in the future the investor will receive a return linked to whatever benchmark is chosen. Several features of the double wa'd (allegedly) make the derivative sharia-compliant: a special purpose entity where the investor's cash goes to avoid commingling, a shariah-compliant asset that is liquid and tradable—such as shares in a