Issue in Subcontracting via MRP

We have set up subcontracting via MRP. Here for the raw materials we use special procurement key 20 or 45. Key 20 creates a PR while 45 creates a reservation assuming stock is available. However we need that system should automatically determine if stock is available and accordingly create a reservation (if stock is available) or PR (if stock is not available). Can we control this via the special procurement key or is there some other field available to do this? I tried with availability check field but does not work. Any inputs on this will be appreciated.