Duren in a S&T for Brown is actually hard to pull off financially. Because of base year compensation rules Dduren would count as only half his outgoing salary for salary matching purposes. So lets assume he got a contract starting at 40 million, he's count only 20 million outgoing. But since Brown makes 57 million the Pistons would have to send out at least 46 million to meet salary matching requirements, meaning they need to a at least 26 million more to a Duren offer. BUT aing 26 million more to Duren means the Celtics are actually taking on 66 million (40 for Duren + 26), meaning the C's would be adding 9 million in salary when they only have about 14 below the tax.
Basically there is only a very narrow range of salaries that would both A) Allow Detroit to salary match an B) Keep Boston below the tax.
Its not impossible, but it would be tricky.