The situations is the lawn has grown incredibly wild with all the Spring rains we've been having, and I want to make sure it is still being maintained during the sale process in case the contract falls through, as well as a general courtesy to the new owners. 

We recently finished renovating a rental property and after a heavy rain it looks like the chimney may be leaking.

If rain is likely and the tenant is aware of leaks, perhaps that is the repair to complete first.

We have one downstairs unit in a 4 plex that leaks water every time there is a heavy rain. 

It was flooded about 2 weeks ago when we got about 5 inches of rain in one morning. 

A vinyl window properly installed will protect the house from rotting out from rain, but an older window may be able to do the same for much less money.

Any water that lands on your property from future rains must stay on your property... at least in most of the muni's I'm familiar with in CA. 

Be patient and don't let your emotions guide your judgement.

@Alisa Gonzalez, is there were water restrictions perhaps the neighbors violated them or maybe they have rain barrels to water with.

If they don't pay by the 6th I file an eviction suit and have a writ of possession and sheriff at the door by the end of the month and a judgement for court costs and punitive damages.