Where the order in which reciprocal promises are to be performed is expressly fixed by the contract, they shall be performed in that order; and where the order is not expressly fixed by the contract, they shall be performed in that order which the nature of the transaction requires.
Illustrations
    (a) A and B contract that A shall build a house for B at a fixed price. As promise to build the house must be performed before B's promise to pay for it.
    (b) A and B contract that A shall make over his stock-in-trade to B at a fixed price, and B promises to give security for the payment of the money. A's promise need not be performed until the security is given, for the nature of the transaction requires that A should have security before he delivers up his stock.