This may have been suggested before, but there are to many mule bug posts to go through.
I was wondering if maybe the cost of renting a mule could be dropped or lowered untill the bug is fixed. I know it may be a hassle but it seems fair to me, as 10 silver lost to a bug is excessive.
Actually the bug is that the NPC creating script doesn't change when it creates a new one. So if someone buys a mule, and someone else buys a mule, if you return mule A, mule B (With the same number) it'll disappear.
Possibly a temp solution if can be done is to only allow one mule to be bought at a time? It would be nice however AND more convenient if can be defined to one area or NPC. In other words if can't buy a mule from Trolls Bane, can go to Varshikar to buy one there without worrying about it disappearing. If this can be placed, a certain max time to have a mule would probably need to be in place to allow others to rent one also.