What is the ruling on selling goods in installments on behalf of the agent, considering their cash price as a debt owed by him to the principal, and is it permissible for the agent to dispose of the exchanged goods, or to set two prices for the commodity, or to purchase it from the principal at a price exceeding the source price?

What transpired between you and your uncle is not an agency agreement. The merchandise is your property, and its price is a debt owed by you. You may sell it in installments or take an old battery as part of its price.

Exchanging a new battery for an old one plus a sum of money is permissible because these are not usurious assets.

It is permissible for a battery to have two prices (when exchanged or sold without exchange), provided that the sale is based on one of the two options and one of them is specified at the time of the contract.

If you asked your uncle to purchase a commodity, and it was merely a promise to buy, then he may purchase it and then sell it to you for a profit. However, if you appointed him as an agent to purchase, then he is not allowed to profit except for a known fee for the agency.