The pet fee or deposit for the move in of 2 cats (you did charge one, I hope) should have just about covered the extra anyway.

I would do between 50-100% or original deposit amount. 

WOW.. 40k plus deposits on a home to be built.. ??? 

If so, you can use the security deposits from both tenants to cover missed rent. 

And add an additional security deposit, but more importantly is "Why".  

Quote from @Aj Green: Quote from @Gregory Wilson: The real issue is how much of a deposit do you have to try to get back from them.Sure, your contingency might work but I assume you gave the deposit to the seller.

Quote from @Clayton Silva: Quote from @Jeffrey Blackman: Quote from @Clayton Silva: 2nd position HELOCS on investment properties are typically capped at 70% LTV and have rates ranging from 9-11% just a heads up unless you use a local credit union where you have cash on deposit with them. 

With a bit of planning and good operations, one can easily manage 20 to 30 properties without an assistant, with less than a day or two a month.There's lots of automated ways to check on rent, make deposits (my tenants deposit directly into a bank account), receive maintenance requests, have maintenance done, etc., etc.One thing I could argue is to have your tenants ONLY email you, especially for maintenance requests, and that email goes directly to the vendor that you have set up to handle it.

The amount of this deposit is going to depend on the auction itself.

But you don’t need a new lease to make a security deposit amendment, that’s just a one page amendment.