Cross currency payments

Hi,

If I enter an invoice in one currency, for example $CAD, in an operating unit set up for $CAD operation that interfaces to a primary ledger in $CAD... Do I have the option to pay the invoice in either $CAD or $USD which is my native currency?  If I do how does the accounting for the $CAD invoice paid in $USD get recorded in my $CAD ledger?  We are going international and nee to get this understanding.  Thanks

Thanks