There is no way to change the currency for the seller of documents. As a seller will usually be targeting their own country, they need to be able to set the currency that they use in their Paypal account or they will be getting charged currency conversion and cross-border rate.
By offering currency at document level, with a default of the global currency set in back-end, this issue can be avoided.
Currency at owner level would be less useful as one seller may want to offer alternate language options in that countries currency, if at document level it gives maximum flexibility.