If the current low ask is $14, the then order is placed as "buy 7.6153 bitcoins @$13" ...if the current low ask is $12 and can cover my entire order, then it will execute a trade immediately, buying $100 worth of bitcoins (or $99/$12 = 8.2500 bitcoins).
Steve, if I understand you correctly - the feature you are requesting is the rule of executing orders at specified or better price (limit orders). Such kind of execution is default at our newly open
imcex.comThe interface of action is pretty close to what you've described. For example, you can Sell an amount of LRUSD at desired price in exchange for Bitcoins. IMCEX exchange will try to execute your order with the specified price (or better if available on market) until the order is either complete or expired. If matching orders cover only part of your - your order will be executed part by part also until the order is either complete or expired.
As I've said we are still in beta and there is a very small volume available, but we will be glad if you decide to stick with us in future.