Let me sign the water leakage compensation agreement downstairs.

Unreasonable. Once the water leaks, he should pay compensation, and you are not obliged to pay compensation. According to the provisions of the Property Law on the differentiated ownership of buildings, the owner's use, income and disposal of his own exclusive part cannot harm the interests of others.

The use of the water pipes connecting the upstairs and downstairs belongs to the "compound right composed of the membership rights formed by the same relationship" in the building ownership, that is to say, it should be used for the same purpose, and you have certain rights to the water pipes downstairs. If you saw the water pipe downstairs without permission, your rights have been violated, and you can ask for "eliminating the influence", "restoring the original state" and "economic compensation".