What is the ruling on a partner developing his co-partner's land (by purchasing a water pump), on the condition that its price be deducted from the landowner's share, in exchange for half of the profit?

The form of musaqat (sharecropping/irrigation contract) mentioned in the question is valid, and it is the view of the Hanbalis. As for the Shafi'is and Hanafis, they do not permit it.

Based on the stronger opinion allowing musaqat for all types of fruit-bearing trees, the cost of purchasing the water pump is to be borne by the landowner, while the fuel necessary for the pump is to be borne by the worker.