You can always add sugar to a dish but you can never take it out. The same goes for salt, pepper and just about any other spice or seasoning. However, if you got happy with the sugar when making curry, you can balance it with bitterness or acidity. Bitter, spicy and sour taste sensations can offset excessive sweetness -- to a degree -- and are the best remedy for cloying curry.
When you want to overcome excessive sweetness in curry with bitter, sour or spicy ingredients, add them to taste and sparingly, or you'll exacerbate the problem. Fenugreek is a common curry ingredient, and has an aromatic bitterness that, when used judiciously, brings a sweet curry back to harmony. To balance with bitterness, add 1/4 teaspoon of crushed fenugreek seeds per cup of curry. Let the fenugreek simmer in the curry for 2 to 3 minutes, and then taste and adjust slightly if necessary.
Red chili flakes bring a lot of heat for their size, and add a pleasant spiciness when you use them sparingly. Add 1/4 teaspoon of red chili flakes per cup of curry, and then simmer for 2 to 3 minutes to extract the spiciness. Taste the curry and adjust with more chili flakes and, if necessary, fenugreek.
Acids, such as lime juice, are the yin to sugar's yang. Squeeze juice from one-quarter of a lime into the curry and stir. Taste to see where the tartness put the sweetness, and add more if needed. A touch of chili flakes along with the lime might be what the curry needs to offset the sweetness.