What is the ruling on an agent completing the price of merchandise from his own money when its price increases, on the understanding that he will be reimbursed the amount and paid his fee upon his return? And is this action and its resulting profit or sale considered void?

There is no objection to that, because the compensatory contract was concluded without a loan or an advance, and then the need for a loan arose afterwards, without the loan bringing any benefit to the lender. What is forbidden is combining a loan and a compensatory transaction that leads to usury. However, if the compensatory transaction is separate and then a loan arises without prior agreement or benefit to the lender, then it does not fall under the prohibition.