Tenants moving out while behind on rent

[David Boden]

We have a 6-plex we've owned for just over a year. All units are under market value in Los Angeles. 
One unit has been giving us trouble and  have been late on rent since before the pandemic and we were looking to begin eviction proceedings right as the moratorium was put in place.

We were considering doing a cash-for-keys option if they didn't pay rent on time this month and we've already waived all the late fees for the last months (per the County). When we wrote on the 1st to ask if they intended to make any payments in September, they wrote --

Hello how are you guys doing ? Thanks for helping us in the best way possible any small gesture has been a huge help. Thing is that due to all this COVID my moms husband lost his job. So we are going to be leaving back to our hometown Sacramento. Thank you for everyThing/ and your patience, sorry for the late notice. Take care god bless and good luck. 

We are at a loss for how to proceed. They didn't give us a move-out date. We are owed roughly $3,500 in back rent payment. We haven't done an inspection (they've had a cat for the last 3 months in a building that doesn't allow pets).

We are looking for advice on how to respond, how to phrase that they aren't entitled to getting their security deposit back and what we can do about the rent. Rent has been coming from the tenants' son who works at a fast-food chain that lost his shifts. Thanks, all!