Visited the Delta site but didn't see an answer to this.. I have a $713 credit and an upcoming business flight for which I will be reimbursed up to $500.

Does anyone know if I can apply part of the $713 to buy an M class ticket for the MQM bonus and end up with a new charge of $500 or so?

The Delta site tells me that if I use the e-credit to buy a less expensive flight, the remainder is still available but that's all.