In this paper I develop an equation for the value of an option to exchange one asset for another a within a stated period. The formula applies to American options, as well as European ones; to puts, as well as calls. Thus, I found a closed-form expression for this sort of American put option and a put-call parity theorem for such American options.
One can apply the equation to option that investors create when they enter into certain common financial arrangements. The investment adviser, who receives a fee which depends at least in part on how will his managed portfolio does relative to some standard, has an option to refuse the fee and declare bankruptcy if the fee is extremely negative. The short-seller has the option to similarly escape his obligations, at the expense of his broker. The offeree in an exchange offer may have the opportunity to exchange one company’s securities for those of another. The buyer of a standby commitment has the (put) option to trade mortgage notes for dollars in the forward market. In each case the value of the option depends not only on the current values of the assets which might be exchanged, but also on the variance covariance matrix for the rates of return on the two assets, and on the life of the option.