That sounds like it would come with a number of tax issues.
Is this money treated as a gift? As consulting? Work-for-hire?
Would it be subject to taxes or social security withdrawals?
Would the reviewer need to give tax info to the publishing company?
If the reviewer does, say, 20 reviews for an Elsevier-published journal, does that exceed a limit where tax reporting laws change?
Who pays for currency conversion?
Can the paper pay for a review by a Cuban or Iranian scientist?