What is the ruling on a man saying to his brother: "If you rent a guesthouse for me and my friends in this city, we will give you 1000 Riyals as a gift from each person who enters this guesthouse"?

The phrasing is ambiguous. If the intention is that your friend will give you a thousand when you find him a resort, then this is a permissible contract of ju'alah (reward for a service), based on the Almighty's saying: "And for him who brings it is the load of a camel, and I guarantee it."

However, if the intention is that every person who enters will give you a thousand, then this is invalid due to jahalah (uncertainty) and gharar (deception/risk), as it is a condition in ju'alah that the reward (ju'l) be known in type and amount.