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.
You must have sufficient current Available Funds to meet the calculated margin requirement for a new order. However, when short selling a stock option the sale proceeds can be included in that position's margin requirement.
Additional information is available in the following IBKR Knowledge Base articles: How do I sell stock short? and How to determine if you are borrowing funds from IBKR?