If you agreed to provide 5 files for a Web slide show and you have provided that to your client, that part of the agreement is fulfilled. If they are asking for the original PSD files, they intend to own the work. In that case it is entirely appropriate to charge for the PSD files and a good deal more than you agreed to charge them for the files for a Web slide show. There is also the copyright ownership issue here. If they want to modify the files then you need to either relinquish the copyright to them or give them rights to edit, modify, change, create derivatives, etc.
Perhaps you may start by inquiring what they need the PSD files for. I assume you may have multiple layers, filters, etc in that file, do they want all the layers intact or do they simply want larger pixel dimension, uncompressed files? Then decide how much it is worth to you to give up your work.