Question

Two situations that happened to me recently, and presumably have happened at other times in my life as well.
A. I bought a computer on eBay, and in order to save on taxes I had it sent to a friend who lives in the United States. In the end, the friend asked to keep the computer for himself and pay me for it so that I could buy a new computer.
That same computer is now being sold at a lower price. Do I need to ask him only for the new price? And if not (seemingly I am allowed to sell it to him at whatever price I want, as long as it is not outrageous), do I at least need to let him know that the price is currently cheaper? (Perhaps he would choose not to buy it from me but instead order a new one for himself.)
B. A friend lost an item of mine and paid me its cost—according to the price on the website I sent him (in good faith). Afterward I found the item more cheaply. Do I need to return the difference to him? Can I buy a different item with the money? (Maybe the friend was willing to pay its full value—beyond the letter of the law—because he thought I needed the item and wanted to be fair to me, but if he finds out that I am buying something else with the money, he will insist on his strict legal rights with me. Asking him is obviously uncomfortable.)
What is the Jewish law, and what is your view regarding integrity in these cases?
Thank you very much.