Personally, I find it inexcusable that a dealer would not have oil filters on hand but, what can I say?
IMO you were not out of line asking for an oil change at 300 miles.
This is the most critical and important change of any that will happen because we all have found that in this first 300 miles there is a
lot of metal chips and debris that collect on the 2 drain magnets, the inlet screen and, I'm sure, in the filter itself.
As you know, these cartridge oil filters are not meant to be cleaned and whether I would trust one that some shop had "cleaned" would depend on how they did it.
If they had a special fixture that forced a cleaning fluid thru the filter body from the outside (clean oil side) towards the inside (dirty oil side) and all of the flushed fluid from the inside was directed away from the filter and filtered I might be willing to accept it.
If on the other hand they dumped the filter into a tank of solvent or oil and sloshed it around there is an excellent chance the harmful metal particles that were trapped on the inside (dirty side) were washed out to collect on the outside (clean side).
Because the outside of the filter is the downstream side, that would place the particles in a location where they could be easily washed into the critical engine parts.
Without knowing how they did it I would be hesitant to run the engine.
If you ordered your filters using the Nfieldgear site you should know that the computerized site often says parts are backordered when they are readily available.
I strongly suggest that you call them directly at 1-800-358-0938 and talk to a real person.
They may be able to get you your filters to you in a few days (unless they truly are out of stock).
On the plus side, if you have any mechanical knowledge at all or even if you don't you have found this web site. We can guide you thru almost any service or repair your bike may need.
That said, you really need to do your own oil filter and oil changes yourself. It isn't difficult and when you are finished you will know the job was done right.
I suggest you follow this link to a post I made describing exactly how to do this little job:
http://www.enfieldmotorcycles.com/forum/index.php/topic,10782.0.html