Indian troops had no real effect on the defence of Paris, so losing them wouldn't affect that. I was, however, suggesting more along the lines of increased recruitment in India to have a larger Indian Army 'at home' rather than denuding the contribution of the Indian Army to the other campaigns of the British Empire.
As others have said, the Germans can't help the Afghans once War breaks out. This means your POD for a victorious Afghanistan requires a stronger Afghan military pre-1914, which IMO would result in a stronger Indian Army in India. Once Afghanistan declares for the CP, they're asking for this stronger Indian Army to invade. So for Afghanistan to gain from Brest-Litovsk (presumably in the form of territory they have gained themselves, as opposed to 'gifts' from the Germans) they're required to defeat Russia and Britain. I find that handwavingly unlikely.
Also, don't forget if your POD has the Afghan militarily involved in Russia in 1914, there's a fair chance Britain will see that as a greater threat to its interests than the areas that they took part in with OTL 'sideshow' operations. Fighting Afghans on the NWF to defend Delhi or sending Indian troops to Gallipoli? I know which one the CIGS is going to chose.