Is delaying the sale of dollars or the delivery of their price after the sale considered usury (riba), given that the dollars will be sold hand-to-hand with the buyer, and the agreement is that I get what exceeds a certain agreed-upon amount?

This transaction is a permissible brokerage according to some scholars. You are an agent and a broker for the dollar owner. The absence of your client from the exchange and the delay in his receiving the money from you do not matter. As long as you exchange hand-to-hand, the exchange is valid, and your separation from your client does not matter.