Pay an expense report invoice in a currency other than the employee's base currency

We are on EBS R12.0.6 currently. We have a US-based employee responsible for overseeing a London office location; the employee frequently travels to London and incurs cash expenses in Pounds.  The employee is set up to receive expense reimbursements in US dollars by electronic transfer, but has requested the option of receiving some reimbursements for cash expenses in Pounds to avoid buying local currency for every trip. We know how the expense report can be submitted with the reimbursement currency set to Pounds, but haven't been able to determine how to pay the resulting invoice since the employee is only configured for Dollar payments.