FRANKFURT — France and Germany may effectively run the European Union, but Finland has demonstrated how even a small country can disrupt their grand designs.

By insisting that it receive collateral from Greece in return for aid, Finland is threatening to upend an agreement that euro zone countries made in July to expand the European Union bailout fund.

That agreement is crucial to restoring confidence among bondholders that Europe can find a lasting solution to its sovereign debt troubles. But the continued squabbling in Europe has alarmed policy makers around the world, and the dispute over collateral provides one more measure of market uncertainty this week as the summer lull ends and trading regains a more normal volume.

Spain and Italy, whose beleaguered economies are receiving support from the European Central Bank, are scheduled to conduct debt auctions this week.