Me and my partner want to engage in a sequence of Bitcoin transactions. The first transaction involves me sending $400 in BTC, and the second transaction involves my partner sending me $900 in BTC. To establish trust, both of us are willing to deposit $900 in BTC into an account, which will be returned to us after the completion of the transactions.
How should we proceed? How do we find a trusted intermediary?
You have some ways to solve this problem, like finding a suitable agent that you both can trust and placing your funds into his account and once the requirements are met the funds will be returned. If you don't trust that dude then set up a multi-signature wallet with each other I hope you already know how it works (if you set 2 keys then to make a TX you need both of them, one will be yours and the other will be your partners) the risk in here is if any of the both lost the key or won't cooperate then things will become bumpy for both of you.
3rd way is if you don't want to do this with each other then use multi sig option with a suitable agent in this way none of the three (you, your partner, and the agent) can fly away with the money. Another way is to use some platforms like LocalBitcoins which I heard has there own built-in escrow service and Paxful too. You can use these or the best option is to prefer an agent from this forum, but check their trusts before making a trade, and fees as well.
Bitcointalk Escrows - Trade Safely! this thread might help you.