Help with inventory control

I need the following to work with PostgreSQL.  I was thinking of reading uncommitted transactions, but just found out they do not work on PG.  I need to have user 1 take an item form inventory for an order and before user 1 finishes the transaction user 2 will query the inventory.  I need user 2 to see the quantity on-hand reduced by the amount user 1 took.

Because read uncommitted is not supported is there a way to do this?

Thank you for all your cooperation.

Fernando