Ways to combine Shimano M-615 SGS with 9-speed cassette and shifter
Shimano 9 and 10 speed mountain stuff have different cable pulls. 

So, if you want to use a Shimano 10 speed mountain  derailleur in a Shimano 9 speed setup (i.e. 9 speed shifter + cassette) you either need to:


Buy a friction shifter, which obviously doesn't care about the speeds
Buy a device which can change the cable pull appropriately (in this case, a JTek Shiftmate 6 would work) and use a Shimano-compatible 9 speed shifter. Note that this will likely cost more than the 9 speed derailleur. 


10 speed cassettes in a 9 speed system introduce another level of complication since the cogs are a different distance apart, and would require a different adjustment to cable pull. It'd be like you cascaded a JTek Shiftmate 2 and 6.