Pet damage question

[Tanya F.]

We have some good tenants that we need to talk to about their pet dog.

We ask for an extra pet deposit ($500) and the damage has already gone above that. We haven't discussed it with them yet, but here's the situation.

The tenants called us and said their air conditioner doesn't work. It's an old unit (from the 70's) and we knew we'd have to replace it at some point soon. We've gotten two estimates so far for a new system. The first estimator pointed out that it's obvious the reason the old unit isn't working, the refrigerant lines have been pulled off the unit (and sadly all the old (Freon) refrigerant has escaped, polluted the atmosphere).   When we were over with the second estimator, we saw that the dog had been chained up to the old unit, so then it became obvious how those refrigerant lines became pulled off. It's a strong dog.

The estimate for a new AC is something like $2500. I asked the service guys how much would it be to refill the old unit with the old refrigerant, and they estimated $700 (old refrigerant is very expensive). We do plan to upgrade, but I asked what a repair would cost because it seems that's the amount the tenants should be responsible for.

We like these tenants, and plan to renew.  But we do need to charge them for the damage to the old AC. Their security deposit is $1100, and the extra pet deposit is $500, so we have $1600 from them, but if we are to charge them the estimated $700 to repair the system (even though we will choose to upgrade instead), should we ask for that payment separately?  This has never come up for us before (that is to say, damage from a pet in the middle of a lease period). We like the humans, and it may have been an honest mistake (though stupid). Probably one of the children decided to tie the dog up to the old AC unit. 

Nevertheless, anyone have any advice for us?  Should we write a letter and ask for separate payment now? I think the $700 contribution to the new AC system sounds reasonable...opinions??  Or should we just plan to deduct from the security deposit, even though they may not move out for several years? Like I said, except for this one instance, the humans and the dog are just fine.