I have following scenario and wonder how to handle it in Sql (In VFP I would have used FLOCK)

Inventory Balance = 100

User A Selects Out from Inventory Table
Uses the 100, Does 'stuff' with Inventory Balance whcih Takes 5 Minutes, and needs to add 5 to the 'Curent ' Balance
(If I use a Sql Update Balance woukd be changed to 105 (100 + 5)

User B Selects out from Inventory Table at same time as user A (Balance= 100)
Uses the 100, Does 'stuff' with Inventory which Takes 7 Minutes, and needs to add 7 to the Balance () (whatever it happens to be now
(If I use a Sql Update , Balance would be changed to 107 (100 + 7)
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What I really want the new balance to be is 100 + 5(User A) + 7 (User B) i.e. 112

Do I still use some form of Locking or is there 'some other way' to do this in SQL

Many thanks in Advance
Gerard