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?
What you need is an escrow service. With that you can send the BTC to a 3rd party who both of you trust and he should send the $900 back.
But i don't get the point at all about what are you trying to do, if this process is for building trust then you don't need the escrow because that breaks the idea about building trust between your partner and you.
Another option is to share a signed transaction as collateral, in the scenario where someone decides to not send the coins back then you can take them by broadcasting the transaction. But it still a double-edged sword.