The value is part of the item properties. Either the wallet/envelope/clip is created as a map item, then the value is set there. Or the item is spawned dynamically via python, then you have to use the SetValue() command in the spawning function.
You can also create it as a map item, and later use the SetValue() function later to dynamically change its contents, possibly due to the character's haggling skill, or standing with the npc who gives the item.
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