Enter the cost basis as "0", and increase the sale proceeds amount by the absolute value of the "negative"cost basis - this will result in the correct gain calculation.
Example:
If the reported proceeds are $100 and the reported cost basis was $(50), your gain would be $150 ----- $100 minus $(50)
Using the workaround, enter the proceeds as $150 and cost basis as $0. Your calculated gain would still be $150 ------ $150 minus $0
You are correct that the program will not accept a negative value for the cost basis; by definition you cannot have a negative cost basis without some equal, offsetting gain.
