Commitment and Availability

Greetings ,

can anyone help me with this case , So now in the manufacturing module if we have on hand quantity=10 and user (A) committed (5) quantities the availability will be (5) , but if user (B) came and tried to commit (8) quantities the system will look on the on hand quantities = (10) not the available quantity (5) , how can i let the system look at the available quantity so when user (B) try to commit 8 quantities he will face an error as the available is just 5 quantities and thanks in advance .