negative and positive for credit and debit respectively. Eg: an entry of -$100 is a credit entry.

Ah, ok. So the "ve" was just a placeholder for the value in the "amount" column for each entry.

How does this handle the fact that credits increase the value of some accounts (e.g. sales, equity) but decrease the value for others (e.g. assets)?

-ve i used to denote credit, not necessarily to indicate increase/decrease. In accounting terms, if you sales increase, your entries on the credit side of sales account increase. In my implementation it means larger negative number. Hope that explains.