[SalesForce] Prevent Recursion of Process Builder

As you were getting a null pointer issue from this trigger, without knowing the line which threw this error, my guess is that you were inserting a Recipt__c object and the trigger.oldMap.get(rec.id).Received__c statement in your code would have thrown a null pointer exception. The oldMap variable is only available in update and delete triggers therefore when inserting a Receipt an exception was thrown. Documentation on trigger context variables can be found in the following link http://www.salesforce.com/us/developer/docs/apexcode/Content/apex_triggers_context_variables.htm

To fix this issue you could implement something like the following

for (Recipt__c rec : trigger.new )
    {
        if(rec.Opportunity__c != null)
        {
            //if the trigger is insert there will be no trigger.oldMap
            //if inserting the object the Opp rollup will be required therefore add the Id to the list.
            if(trigger.isInsert)
            {
                oppId.add(rec.Opportunity__c);
            } //if updating a record check to see if the two values to rollup have changed, if so add Opp to the list
            else if (trigger.isUpdate && (rec.Received__c != trigger.oldMap.get(rec.id).Received__c || rec.Due_Amount__c != trigger.oldMap.get(rec.id).Due_Amount__c ))
            {
                oppId.add(rec.Opportunity__c);
            }
        }
    }

in place of

for (Recipt__c rec : trigger.new ){
    if(rec.Opportunity__c != null && (trigger.isInsert ||  trigger.isUpdate|| trigger.isUnDelete || trigger.isDelete|| rec.Received__c != trigger.oldMap.get(rec.id).Received__c || rec.Due_Amount__c != trigger.oldMap.get(rec.id).Due_Amount__c ))oppId.add(rec.Opportunity__c);


}

See Triggers and Order of Execution for details, but in summation, workflow rules, assignment rules, etc are skipped in recursive updates. This means that workflow rules on parent and grandparent records won't trigger, but before and after triggers on parents and grandparents will, which means you should place your logic within code if you want it to always be evaluated.