Maurice Schweitzer, a researcher from the University of Pennsylvania, put 262 young adults in a situation to test ways to restore trust.  The findings are amazing and apply directly to what you, as an MGO, must do when facing a situation where lost trust with a donor needs to be restored.
You can read all the details in the paper Organizational Behavior and Human Decision Processes, Promises and Lies: Restoring Violated Trust (Maurice Schweitzer, John Hershey and Eric Bradlow, 2006).
Here’s the short summary, stated from the point of view of a non-profit and a donor.
If the donor expected one thing, but another thing happened, the donor will see the difference as an untrustworthy action and most likely will assume that the organization/MGO purposefully set out to deceive her.
If the situation did not involve a lie or it did not have intent to deceive – in other words, it was just a mistake – then it is possible, through subsequent trustworthy actions, to regain trust.  But this path will take longer.
If, however, a promise is made to correct the situation and that promise is delivered on quickly, trust can be restored very quickly.
Here’s the surprising thing.  Schweitzer measured the effect of an apology and found that it failed to influence the trust recovery process at all.  AND, if an apology was given in a situation where there was intent to deceive, it actually harmed the process even more.  The donor just suspected another deception and all it did was deepen the mistrust.
So, if you make a promise, then break the promise, a donor will believe the promise was a lie and that the organization’s intent was to deceive. If a donor believes the deception was deliberate, then trust is permanently harmed.  So one obvious principle here is: don’t make a promise you can’t keep.
OK, here is how all of this works and what Jeff and I suggest you should do when you face a situation where trust is broken and you need to restore it:

1. First, do not make a promise you cannot keep.  This happens too frequently in major gift fundraising where an MGO represents something in the giving transaction that he or the organization may not be able to deliver on.  This happens because the organization has not done the work it needs to do in creating offers for MGOs to present to donors, thereby leaving the MGOs with the task of coming up with content themselves.  Be careful here.
2. If a promise has been broken, speak directly and honestly with the donor, confessing that you are aware of the broken promise.  If you know what happened, tell the donor.  Do not hold back.  This may get delicate.  I know of one situation Jeff and I dealt with where a sizable donation was meant for a program that didn’t quite come off as represented.  No one set out to be deceptive in this situation, but it sure looked like it from the outside.  The MGO had to meet with program staff and the executives of the organization to figure out a way to explain the thing with integrity.  They got through it, but it was a wild and scary ride.
3. If you do not know what happened, promise to find out – and give a deadline.  Then go find out and get back to the donor.  Remember, though, once you have made a promise, the longer you wait to keep it, the more entrenched the donor will get into the place of mistrust.  We had a situation recently where a donor sent in two checks, one for $10,000 and another for $5,000 that got lost in the mail. No one knew about it and then the current MGO, asleep on the job, neglected to follow up when the regular giving went missing.  Can you imagine what the donor was thinking?  When a new MGO came in, the gap in giving was noticed. The MGO called the donor, discovered the lost donation and restored the situation.  The donor was so happy about the way things were handled that she gave an additional $50,000!  Trust regained!
4. An apology is only good if it is attached to a promise.  Remember this.  It is not good enough to just apologize.  An apology just by itself actually hurts the relationship.   If you are going to apologize about a situation that has gone bad, be certain you can find out what happened and assure it won’t happen again.  Do this before that apology comes out of your mouth.

Lost trust is a delicate thing.  We all have experienced it and it hurts.  But trust can be restored if we deal honestly with our good donors, telling them what happened and promising to make things right.
I hope this series on trust in major gift fundraising has been helpful to you.  I know that in writing it I have been reminded once again what a delicate thing the giving transaction is and what an important role trust plays in it.  It has also been good to be reminded that I, myself, have to continually work on being a trustworthy person in all of my relationships, and that it is always good, in all aspects of my life, to walk in the light.
Richard