[Updated August 9, 2013: I bow to the wisdom of the commentariat! "Acme shall purchase" it is.]
Acme and WidgetCo enter into a contract providing for Acme’s purchase of 1,000 widgets. WidgetCo delivers the widgets to Acme, along with an invoice. Acme pays the invoice.
In that scenario, when does Acme purchase the widgets? I propose having the contract say, “Acme hereby purchases” the widgets, rather than “Acme shall purchase” the widgets. It’s standard commercial practice to refer to someone’s buying something when the decision to purchase is made, even though delivery and payment follow later.