I am glad that you mentioned the ecumenical council vs. local council question. I see that you've tagged it with usury, so I hope you will accept answers based on usury and not just interest in general.
Local Councils
There are a number of local, pre-schism councils that prohibit usury by laymen. Because they are pre-schism they can be understood to be "Eastern Orthodox," but none of the cities in question are particularly Eastern, and again they were local.
In the 4th century there was a Synod of Elvira which predates the Council of Nicaea (source 1 with original Latin, source 2).
One of the canons issued at that Synod says:
If any clergy are found engaged in usury, let them be censured and dismissed. If a layman is caught practicing usury, he may be pardoned if he promises to stop the practice. If he continues this evil practice, let him be expelled from the church.
There are quite a lot of things in the canons from Elvira that the Orthodox church does not follow, such as "pictures are not to be placed in churches" and clerical celibacy. When responding to the iconoclastic claim, one modern-day Orthodox writer has said:
This canon (among the rest at Elvira) were neither ecumenical in scope, nor eternal in application. Like many other encyclicals, epistles, and synods in the history of the Church, the scope was both locally and historically specific.
(For example, Christianity was not yet legal in the Roman empire, so you could see how that would change things.)
Other pre-schism local councils forbade this practice as well, e.g.:
- "The synod of Pavia in 850 excommunicates lay usurers and prescribes full restitution to their victims" (source, page 16 of the paper). The 850 Synod of Pavia is so obscure it doesn't even have its own Wiki article.
- "The 12th canon of the First Council of Carthage (345) and the 36th canon of the Council of Aix (789) have declared it to be reprehensible even for laymen to make money by lending at interest." (source)
- A few more sources here
Here is a page that lists Christian writings on usury throughout history. I know the site does not look pretty, but the pages it links to are generally sourced.
Saints
Church fathers also wrote against usury; Gregory of Nyssa did so perhaps most thoroughly. (See a homily on the subject.)
Pope St. Leo the Great (who, being 5th century, is also an Orthodox saint) also spoke against usury in his sermons (source), which I believe were given while he was Pope:
the money-lender's trade is always bad, for it is sin either to lessen or increase the sum, in that if he lose what he lent he is wretched, and if he takes more than he lent he is more wretched still
This isn't as authoritative as a council, but I'd certainly think he was speaking ex officio.
