I'm working on a gift certificate system and would like putting together a schema and need a little help as to do it.

Gift Certificate would have unique numbers
then obviously the easiest scenario would be if the person used up the whole amount but i'm thinking if someone used partial..

So there would be a debit and credit column .. probaby could be done with 1 table? let me know your thoughts

Code:
id     |     debit     |     credit     |    date_modified   
-----------------------------------------------------------------
434   |      0          |     50         |      09-02-2013 11:15:36
434   |      15        |     35         |      09-02-2013 12:12:54