It is narrated from Amr bin Shu'aib (may Allah have mercy on him) that the Messenger of Allah (ﷺ) used to fix the price of camels for the people of the town at four hundred dinars or its equivalent in silver (dirhams), which would be divided according to the price of camels. When camels became expensive, he would increase its price, and when they became cheaper, he would decrease its price. The people of the town are obliged to pay the price that is current at that time.
Hadith Referenceمسند الإمام الشافعي / كتاب القتل والقصاص والديات والقسامة / 1646
Hadith Takhrijاسناده ضعيف لاعـضـالـه ، ولعنعنه ابن جريج ولضعف مسلم بن خالد، أخرجه البيهقي: 76/7ـ 77 وفي المعرفة السنن والآثار له (4891)۔