Your client has come to the realisation that doing changes even inhouse has a cost associated with it. Perhaps even that your cost is not high after all. Thing is you need to deliver to bring in money. Somebody with this as a side job may take very long to do the same. This is a good example as to why not burn bridges, even if it seems like the other party is terminating the business relationship.
Just bill them for the change. You should strive clarify the terms of future conditions, since client clearly was unhappy with previous arangement.
This is a good place for you to review what your contract looks like in relation to releasing sources. For example is it really your job to archive files for them? If not do you bill them a archivial fee, or ask them to supply source files? So look at this a way to improve the way you handle this kind of thing next time even if this one is less than perfect.