What do you do?
You have just eaten a meal at a restaurant you wouldn't be in, except that you got a gift certificate. (a gc that was paid for, not a comp) Your gc is $60, your bill was $51. You put the gc in the envelope. The waitress brings it back with a brand new gc for $9. You will not be eating in that restaurant again. What do you do about the tip?
If you leave the gc, will the restaurant give her cash at the end of the evening? If not, can she substitute it for cash the next time another patron pays in cash? Or have you effectively stiffed her?