Amir al-mu'minin used to write to whoever he appointed for the the collection of zakat and charities. Ash-Sharif says: We have recorded a few portions of it here to show that he always erected the pillars of right and created examples of justice in all matters, small or big, delicate or serious.
Move on with the fear of Allah Who is One and has no partner. Do not frighten any Muslim. Do not pass over his lands so as to make him feel unhappy. Do not take from him more than Allah's share in his property. When you go to a tribe, you should get down at their watering place instead of entering their houses. Then proceed towards them with peace and dignity till you stand among them. Then salute them and do not be remiss in greeting them, then say to them, "O servants of Allah, the vicegerent of Allah and His caliph has sent me to you to collect from you Allah's share in your properties. Is there anything of His share in your properties? If so, give it to His vicegerent."
If someone among them says "No", then do not repeat the demand. If someone speaks to you in the affirmative, then go with him without frightening him, threatening him, pressuring him or oppressing him. Take what he gives you such as gold or silver (coins). If he has cattle or camels do not enter upon them save with his permission, because their major part is his. Therefore, when you get there do not enter upon them like one who has full control over them or in a violent manner. Do not scare any animal, do not tease anyone and do not let the owner feel grieved about anyone.
Divide the property into two parts and let the owner choose one. When he has chosen do not object to it. Then divide the remaining into two parts and let him choose one and when he has chosen do not raise any objection. Continue like this till only that much remains which is enough to satisfy Allah's dues. Then take Allah's due from it. If he disputes your action allow his views, then mix the two (separated) parts and repeat what you had done before till you take Allah's due from his property. 
Do not take an old, decrepit, broken-limbed, sick or unsound animal. Do not entrust the animals (for custody) except to one whom you trust to take care of Muslims' property till he hands it over to their chief who will distribute it. Do not entrust it to anyone except he who is a well wisher, God-fearing, trustworthy and watchful, and who is not harsh on Muslims' property, nor makes them run too much, nor tires them, nor labours them. Then send to us all that you have collected and we shall deal with it as Allah has ordered.
When your trustee takes over (the animal) tell him that he should not separate the she-camel from its young and should not milk all its milk because that would affect its young, and also that he should not exert it in riding. In this matter, he should behave justly between it and all its companions. He should allow rest to camels (who are tired), and drive with ease those whose hoofs have been rubbed off. 
When you pass a water spring stay the camels there for drinking and do not take them away from vegetated land to barren paths. He should allow them rest now and then, and give them time near water and grass. In this way, when they reach us by leave of Allah, they will be fat with plenty of marrow and would not be fatigued or distressed. We will then distribute them according to the (commands of) the Book of Allah and the sunnah of His Prophet (peace be upon him and his progeny). Certainly, this will be a great source of reward for you and a means to secure guidance, if Allah so wills.