I propose three sections. In the first section: a box in which you can put what you want to purchase, a place where you put what you pay for the merchandise, a box where you can drag out your purchase. Second section: an inventory-style set of things you can purchase. Third section: your inventory. Drag the item you want to buy to the first box, put your payment in the second box. The item will show up in the third box. If you choose to drag it out of the third box, your payment disappears from the second and replaces the item in the chest.
A similar UI could be used to create proposed transactions.
See below for an example mockup.