Splitting property up thats under finance

[Bongonut]

Sorry, have a regular account I post from but this needs to be anonymous for a couple of reasons.

A friend has a freehold flat, there is a garage with it.

Friend is selling the flat but owes me money and has agreed to give me the garage, there is a single mortgage on the flat/garage.

Flat alone will sell for 40k over what finance is secured on it.

Roughly how do we go about this? Obviously will be handled by solicitors when the time comes but I'd like to have a decent idea on if things are a hassle, have no experience with anything like it so sorry to sound stupid.

Thanks