Premium (praemium) might also refer to the act of simony. However, it would be strange for this canon to accept simony as long as it was accepted by the local clergy, since the same council forbids simony altogether (see [1474]).
Since abbots and presbyters from the first two sentences of this canon seem to be described by a joint term "presbyters" in the last sentence, it appears that those abbots were presbyters as well. Some of them were likely in charge of the specific basilicas and the clerical communities ascribed to them.