The dwarf assumes everyone will follow contracts to the letter
After all, he does, as do his fellow dwarves. He lays out exacting, precise specifications on the placement of posters, number on each street, positions that they should be hung, etc. Each contract contains the strongest possible oaths permissible in dwarven law that the contract will be completed to the letter, or the money returned. In dwarven society, it would be a crime worthy of exile to break a contract such as this. He assumes that, even if the young humans he hands his flyers and coins to do not carry out the task, they will return the money and the flyers, and he can simply hire new workers.
The street urchins he hires, however, are illiterate and have no understanding of the instruction packets he provides, and, as they can't follow the plan, slap up a few flyers around where the dwarf normally walks, and decide to sell the rest as paper waste. It ends up being used as toilet paper in several local taverns, where adventurers who opted, unwisely, for the fish of the day all have an extended chance to read it.
In this way, the motley adventuring party, looking somewhat worse for wear, arrives at the time and place specified inby the flyer the next day.