Without going too deep Mel, there are some firms that will review DMP files after, say, 6 months with a view to looking at possible IVA proposals. Perfectly legitimate and above board of course as there may be a number of reasons why an IVA was not viable at point of initial contact, but may be viable later on.
If it is a DMP company solely, the fee may consist of 2 further payments on the DMP being held, which is still running, as a fee whilst an external IP looks at the IVA proposal. If the IVA is approved those 2 payments can be viewed as the "transfer fee", over and above any payment they receive from the IP as an introducer. Some companies, of course, will not charge that and view their fee from the IP as sufficient.
If it is a company that provides both DMP and IVA, some firms will charge a flat fee, possibly £100 to £200 to switch solutions internally, ostensibly to cover extra admin.
Each company must have its own business model of course, and all of this is fine if the client is happy to pay these fees.
Regards.