The sender has all 4 keys and can have the destination address, but does not have the tx_key, is there any way to find out, how much money he sent in a transaction using any blockchain explorer?
Or can the sent amount be found out only by the recipient?
Using your spend secret key, you can calculate the key images for all your outputs. Using your view secret key, you can see their amounts. When you check the outgoing transaction in question, you can see which outputs of yours were spent by looking up the key images. This gets you the amount.
A block explorer does not have your secret keys, so is unable to see this.
