Here's what Yahoo rules say:
Please be sure to keep tabs on the trades that you have offered to other managers. So long as a trade is in the offered state, it can still be accepted. Therefore, there is a possibility that a trade is accepted after a player is injured, demoted, etc. Yahoo! Sports will not intervene in such events.
http://help.yahoo.com/help/us/baseball/ ... es-17.html
I had that happen to me as well. I dealt Burrell, Borowski, and Brower for Bonds after news came that he had another surgery. I don't mind too much actually, because I might've done the deal even with the additional surgery.
Is it technically against the rules? No. But it's kind of low. This sort of action rewards the person who gets to the computer faster. Some people just can't get to the computer every second to cancel trades and the like. It's sort of the matter that the waiver system was created to stop...instead of the first come/first serve basis. Explain your situation to the commish as well as the other party and see if you can get an understanding there. If that doesn't work, you could try convincing the other owners why it's vetoable.