Selling properties while owing another player
You can sell houses and hotels to the bank (at 50% value).
You can sell empty property to another player even if it is mortgaged.

SELLING PROPERTY... Unimproved properties, railroads and utilities
(but not buildings) may be sold to any player as a private transaction
for any amount the owner can get; however, no property can be sold to
another player if buildings are standing on any properties of that
color-group. Any buildings so located must be sold back to the Bank
before the owner can sell any property of that color-group.
Houses and
hotels may be sold back to the Bank at any time for one-half  the
price paid for them. All houses on one color-group must be sold one by
one, evenly, in reverse of the manner in which they were erected. All
hotels on one color-group may be sold at once, or they may be sold one
house  at a time (one hotel equals five houses), evenly, in reverse of
the manner in which they were erected.
MORTGAGES... Unimproved
properties can be mortgaged through the Bank at any time. Before an
improved property can be mortgaged, all the buildings on all the
properties of its color-group must be sold back to the Bank at half
price. The mortgage value is printed on each Title Deed card. No rent
can be collected on mortgaged properties or utilities, but rent can be
collected on unmortgaged properties in the same group.
In order to
lift the mortgage, the owner must pay the Bank the amount of the
mortgage  plus  10% interest. When all the properties of a color-group
are no longer mortgaged, the owner may begin to buy back houses at
full price. The player who mortgages property retains possession of it
and no other player may secure it by lifting the mortgage from the
Bank. However, the owner may sell this mortgaged property to another
player at any agreed price. If you are the new owner, you may lift the
mortgage at once if you wish by paying off the mortgage plus 10%
interest to the Bank. If the mortgage is not lifted at once, you must
pay the Bank 10% interest when you buy the property and if you lift
the mortgage later you must pay the Bank an  additional  10% interest
as well as the amount of the mortgage.
BANKRUPTCY... You are dec