If possible, try not to go down the "support Postgres as well" route.

I was in your situation, where client wanted SQL Server since they already have the license. During development, I use PostgreSQL instead, to "support Postgres as well".

At the end, roughly one-third [1] of the total development effort was spent on overcoming SQL Server's limitations, things that you would never have to think about in PostgreSQL.

So, try telling the client that they already have PostgreSQL license as well, with unlimited future upgrade.

[1] This figure was pulled from ass. The actual productivity loss could be more due to similar reasons outlined in http://news.ycombinator.com/item?id=3784750