try http://www.ee.surrey.ac.uk/Contrib/manx/famhist/genealgy/deeds.htm
for a brief overview
Mortgage when paid were cancelled - on default the person giving the mortgage could claim the whole property even though it might be worth more than the loan (the law was changed c.1840's but not before the Gawne's of Kentraugh had acquired vast tracts via this device).
Sale was just that but by Manx law once a property had decended by will it had to pass to the eldest son unless agrrement of all parties to its sale was obtained; agreements were usually Marriage agreements in that property was gifted to the couple usually with a proviso that the givers (parents) were looked after or could remain on the land until death.

The number refers to the actual deed in the records - these deeds are not microfilmed and are probably the only original material encountered by family historian when using the Manx Museum