Est. cash balance means the total cash in cash account. Including available cash and non-available cash, non-available cash may include unpaid cash for purchase orders, cash paid for submitted withdrawal.
The single currency cash amount represents the cash value of the account.
When you open a long position in stocks/options denominated in that currency, the cash value will decrease. Conversely, when you close a long position on the underlying currency, the cash value will increase.
New deposit will increase the cash value, and withdrawal will reduce the cash value.