I think you are right to take the SD if the following is true.
1) The lease you have with the tenant states the correct amount which would be $100 more than what they paid.
2) You could show in your books how you took their rent payment and then applied $100 of the deposit toward the current rent due each month.
3) Your state allows you to use the tenants deposit for rent. If it does not then I would hope that you have a signed copy of the agreement and possible receipts showing the deduction which would show their agreement of this senario.
If it is not allowed to use the deposit for rent in CA then without a signed agreement I am not sure. The question a judge might pose is why you did not have some type of acknowledgement from both parties that their rent is reduced as long as the hold up their end of the contract.