I have some paper CS certs (type B) in front of me and in section 10 it states:
The certificate discount may not be reapplied towards the purchase of another ticket when exchanging or refunding your original ticket, except when the original ticket qualifies for a reduced fare (guaranteed airfare rule applies).
It is profile code S*PMO/DollarUA. If this is the type you have you should be able to refare and the amount of the cert would apply.
I see
jhayes_1780 types faster than I do.