Sugar in Indian curries?
There is nothing inauthentic about using sugar in an Indian dish, even a savory one.  For example, Gujarati cooks often add raw sugar (jaggery) to daal and curries. Quoth Wikipedia:

"It is common to add a little sugar or jaggery to some of the sabzi/shaak and daal. The sweet flavour of these dishes is believed to neutralize the slightly salty taste of the water."

And here's an example Gujarati potato curry recipe, which includes a tablespoon of sugar.

As for how much to add, that's a matter of your taste and the level of acidity and/or saltiness in the dish.  My Gujarati friends in Boston like stuff rather sweeter than I would personally prefer, so be cautious.