Reserve stock against one particular sales order and issue the same

Hi,

My client wants that he should he able to reservestock for a particular sales order only, so that during availability check, the stock is not issued against any other sales order.

I have tried to do stock reservation by using t-code MB21, then the stock gets transferred from Unrestricted stock to restricted stock.

When i Do the PGI, it is still reducing the unrestricted stock and the reserved stock is unaltered.

My client is not a manufacturing unit, they are basically into only trading of goods.

Can some one please suggest what steps can i do?

Also if tehre is any alternative to accomplish the same?

You can mail me any documnettaion regarding the same, is any on priyanka.m24@gmail.com

Thanks'

Priyanka