Hi Rubab,
Below are the different ways i thought your request can be moved forward.
1. Restrict through authorization as suggested earlier.
2. Using User Status, restrict all posting if the plan cost is different from the day when the order is moved to REL. Activate the user status based on approval. This can be similar to your release strategy design but after release.
3. Using User Status, if there is a change in plan cost after release, freeze the transaction, unless the specific approver change the user status manually or you need to design it automatically once he approve it
4. Convince the client, the release strategy will not work for the released orders. In such case, whether they are ready to create a new order else You can develop a report giving the difference of cost at release and at TECO, so this can be their KPI's.
There will be multiple solutions can be use and it is easy from the functional perspective, but your abaper only can convey whether technically it is possible.
Most of the clients wanted everything they like to be in system, as a consultant you should be able to convince the client and should not be accepting each and everything. (My personal opinion)
Regards
Terence