In accounting we create an "allowance" account. So your example would be "allowance for property taxes" this would be considered a contra cash account.
Adding an amount to this is considered a credit, and decreases your cash which is a debit balance.
Debit | Credit
Leaves you with 4000 in available cash.
This would mean that you have 4000 in available cash, that other 1000 can be placed in a savings account. If you actually have a savings account, just make it contra to the savings account. You can have as many as you like. Say "allowance for property taxes", "allowance for vacation". You deposit money into the savings account and credit the allowance accounts for the amounts that you want to allocate that money for. In this context the available cash in the savings account should always be zero, because its been allocated to an allowance.