I have two objects Distributions and distributions used. there is a lookup from distribution to the distribution used object.

I have a field named Distribution amount left on the Distribution object which I want to update everytime distribution used object's field amount Reinvested is populated

So the formula would be 

Distribution amount left (distribution field)= Distrbution Amount(Distribution field) -  Amount Reinvested(Distribution used field)

Can someone please help with the below trgger. Also I want the error message if the amount left is negative. I know my trigger is totally screwed up.

Many thanks

trigger dda on Distribution__c  (before insert, before update)  {
 Map<Id,Id> userMap = new Map<Id,Id>();
    for (Distribution__c item : Trigger.new)
    {
        
        userMap.put(item.Distribution_Amount_Left__c, null);
        userMap.put(item.Distribution_Amount__c, null);
}

for (distribution_used__c item : [SELECT Id, Amount_Re_Invested__c FROM distribution_used__c WHERE Id IN :userMap.keySet()])
    {
        userMap.put(item.Id, item.Amount_Re_Invested__c);
    }
    
    for (Distribution__c item : Trigger.new)
    {
        item.Distribution_Amount_Left__c = userMap.get(item.Distribution_Amount__c)- (userMap.get(item.Amount_Re_Invested__c));
                              
    }