I've given part of the donation back on several occasions, and have declined a tip a few times as well.
Had an appointment where, not to put too fine a point on it, the guy was a straight up asshole. About 15 minutes into the appointment it was obvious that we were just NOT clicking. I got off the bed, picked up the envelope, split the donation in half and gave him back half of it.
There have also been several situations where a client planned a longer appointment, but for one reason or another, wound up staying far less than the time he paid for. When this happens, I always adjust the donation to a shorter time frame - one hour paid for, but he stayed a half hour, I adjust to my half hour rate, etc. Not only is it only fair to do so, but this is just one of the reasons why my regulars remain regulars. I value my regulars highly, and make every effort to demonstrate that they are important to me.
On several occasions, a client has brought the full donation while I was offering some sort of special. I always let a client know up front if I"m running a special, especially if he is a regular and I know he may not be aware of the special. I always let them know they can take the envelope back and adjust the donation. Again, I like my regulars, and want them to stay regulars
I am happy to accept a tip if one is offered, but on one occasion the gentleman laid down an additional $100 on top of the donation. While extremely gratifying, that is way too much for a tip. I folded it up and put it back in his pocket, and told him my favorite kind of tip is simply a good review and a return visit