É um acordo/negociação feito com o banco/senhorio para que o inquilino desocupe a habitação entregando-lhe as chaves contra a aceitação de dinheiro em numerário que o banco/senhorio lhe oferece.
Este tipo de acordo é feito quando, por exemplo, se pretende evitar "maus" inquilinos.

A cash for keys offer is a deal which a bank may make with a homeowner, in which the homeowner is given a cash settlement in exchange for vacating his or her foreclosed home. This type of arrangement is sometimes made to renters, too. The advantage, from the point of view of the bank, is that it gets people out of the house quickly, and the house is often left in better condition than it would be in the event that an eviction was needed. It is important for people to be aware that this type of offer is usually a last resort, however, because once the paperwork is signed, the homeowner or tenant typically has no recourse.
http://www.wisegeek.org/what-is-cash-for-keys.htm

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Note added at 40 mins (2015-04-14 21:20:11 GMT)
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Parece-me ser próximo de uma "execução de hipoteca".

What is Cash for Keys?

If foreclosure is imminent, and you are considering a deed in lieu of foreclosure, some lenders are willing to offer “Cash for Keys,” whereby the lender will actually pay you to vacate the home in a timely fashion. The money you receive in exchange is intended to pay for your relocation costs.
How it works

It is in the bank’s best interest to forego the cost and time involved in carrying out a foreclosure. When the bank agrees to give you cash to move out on a mutually agreed upon date, you agree to maintain the property and leave it in a “broom sweep clean” condition. Often the cash is paid upon or after your exiting the property.
http://homeownership.org/cash-for-keys/