Just bought a new SLX front mech, but bought a 10-speed by mistake. Can I still run it with my 9-speed XT rear mech?

Can anyone forsee any problems?

Wouldnt have thought so. You can run 10spd kit on a 9spd chainset.
(Or at least I’ll find out soon enough).
😉

Eh, doesn’t make any difference what the rear is, fronts are either 2 or 3 ring mechs. I’m actually running a 3 ring mech on a 2 ring set up too just by winding in the travel limit screw.

The 10sp front derailleur will be slightly narrower to match the narrower external profile of a 10sp chain. So running that derailleur with a 9sp chain will make it more prone to rubbing. Not such a big deal if you have a double and bash, but will be more of an issue with a triple.

Interestingly, the front mech is 10 speed specific but the shifter isn’t…

The internal diameter of a 10sp chain is the same as a 9sp, hence the chainrings will be in the same place in relation to each other, which is why the shifters are the same.

rings are closer together on 10sp shimano chainsets than 9sp ones, this may cause an issue

can you return it?

If you run a 9 speed cassette with a 10 speed chain,on a 10 speed chainset and a 10 speed dynashift front mech.. you can shift with a 9 speed l/h unit and have no problems.(so 9 or 10 will be fine for left hand shifters)

Because the 9 speed cassettes width is the same as the 10 speed,and the width of the chain coupled with the distance between the front mech plates has been designed to work together,you wont experience excess rubbing in the extremes.