Anusha,
Three solutions immediately come to mind:
1. When sales order is entered, it remains unconfirmed if there is inadequate existing supply (do not use TRLT). Then, once a day or so, run MRP on finished goods. This creates planned orders for the FGs. Now run capacity leveling. This creates a finite FGS supply plan that is feasible. Firm the resulting planned orders. Now, run V_V2 against the FGs sales orders, confirming them them against the Finite feasible production plan (firm planned orders). This provides a capacity based availability check that is as detailed as you care to design. The down side is that immediate confirmation is not available unless there is existing excess FGs supply. Also, the combination of the MRP run, the Cap leveling run, and the Rescheduling run are very intensive computer processes; you may not have enough time or computer horsepower during a working day to complete them.
2. Strategy 62. Create your planning materials so that a planning material represents the bottleneck capacity resource. When the forecast is created, the planners must constrain it to the capacity before actually entering the PIRs into the system. Then, all FGs sales orders can only be confirmed if there is sufficient unconsumed forecast against the planning material. Since the planning material represents bottleneck capacity, this is in effect an ATP against bottleneck capacity. The down side of this is that strategy 62 ATP does not respect FGs or Production order, it only checks against forecast. Also, the cap check is just against bottleneck; that may not be enough for you.
3. I participated in a design team that created a solution (but never implemented) where Product Allocation was used as a pseudo capacity constraint. The design was very promising, but I never saw it actually running in production.
Some of the other SCN experts may have some other workarounds as well.....
Best Regards,
DB49