Sorry, I don't think I was being clear with what I meant. If he threw the $500 in on top of the $100 and then picked up the $100 chip, then yes I agree with that being a string.
What I meant by that was if the player has $500 in his hand, reaches out towards the $100 chip and with the $500 chip still in his hand picks up the $100 chip. He then releases both chips at the same time to show that it is $600. This motion can be done fluidly in a way that makes it very clear what the action is. If he had released any chip into the pot first then I would call it a string.