So I cannot seem to figure this one out. I do not use maps all that often and I am having a hard time determining how to update a value from it.

I am trying to update the inventory by subtracting the value from the map. Pleas see the code below:

trigger PO_Request_Update_Inv_Transit on PO_Request__c (after update) {

	PO_Request__c[] porequest;
	
	Set<Id> poSet = new Set<Id>();
	Set<Id> podetailSet = new Set<Id>();
	Set<Id> inventorySet = new Set<Id>();
	Map<String, Decimal> popartMap = new Map<String, Decimal>();
	List<Inventory__c> inventoryList = new List<Inventory__c>();
	List<PO_Request_Detail__c> porequestdetailList = new List<PO_Request_Detail__c>();
	
	for(PO_Request__c po:porequest)
	{
		poSet.add(po.Id);
		Decimal invQty = 0;
		
		If(po.PO_Req_Status__c == 'Approved')
		{
			porequestdetailList = [Select Id, PO_Request__c, Part_Num__c, Qty__c from PO_Request_Detail__c where PO_Request__c IN : poSet];
			
			for(PO_Request_Detail__c prd:porequestdetailList)
			{
				popartMap.put(prd.Part_Num__r.Id, prd.Qty__c);
				podetailSet.add(prd.Part_Num__c);
			}
			
			inventoryList = [Select Part_Num__r.Id, Qty__c from Inventory__c where Part_Num__c IN :podetailSet];
			
			for(Integer i = 0; i < inventoryList.size(); i++)
			{
				if(popartMap.containsKey(inventoryList[i].Part_Num__r.Id))
				{
					inventoryList[i].Qty__c = inventoryList[i].Qty__c - 
				}
				update inventoryList;
			}
		}
	}
}

 My main problem is how to get the mapped value and use it in my calculation to update.

inventoryList[i].Qty__c = inventoryList[i].Qty__c - ???

Thank you for your help.