What determines how much cash is Available for Withdrawal in a Cash account?

Your cash balance in this currency is most likely not settled yet and that is why the withdrawal will created a margin loan. If you have recently executed a currency conversion or closed a position, the cash withdrawal of the proceeds will increase your margin loan on your margin type account until the trade has settled. This could take up to two (2) business days (T+2)…

You will earn interest at IBKR's published rates based on your total settled cash balances, regardless of where your cash is held.  

Interest on Insured Bank Deposit Sweep Program balances will accrue daily from the day funds are deposited into a program bank through the business day preceding the date of withdrawal from that bank. Interest will be credited monthly. The specific program bank assigned to your account does not impact the interest rate you receive…

You cannot simply use cash received from a short sale to meet the margin requirement for a new order in your account. A short stock sale is a margin transaction in which the account holder is borrowing stock rather than cash. While the proceeds from the short sale are credited to the cash balance of the account, these funds must be posted with the lender of the shares as collateral to secure their return. As a result, and in recognition of the fact that the loan transaction is subject to its own financing terms, the cash collateralizing the loan is excluded for the purpose of determining whether a margin loan exists…

Loaned shares may be sold at any time, without restriction. The shares do not need to be returned in time to settle your sale of the share and proceeds from the sale are credited to the client’s account on the normal settlement date. In addition, the loan will be terminated on the open of the business day following the security sale date…