Quite right, you cant bring the ARC in line with the annual but the ARC should only be paperwork if the maintenance is planned correctly.
I assume these are Slick mags you are talking about? if so you need to ask why the subpart F company sent a set of mags that were so near overhaul for a 500 hour inspection.............. it is just a daft thing to do and it is costing you money. It is not an EASA thing it would have happend on the BCAR if the maintenance company had done this.
I think you must ask the maintenance company to fit these mags for free as if they had chosen to think ahead thay would have only fitted these mags once!