How to report dishonest seller?

Been eating at a mala hawker near Clementi MRT. It's a Chinese owned mala stall, you pick your food and weight, and there are different price for veggies, mushroom, and meats.

I go there for dinner once. It cost for the whole things is pricy and unbelievably. Which if I take the price divide by the meat price per 100g (the priciest among the category), it was almost 700g, though I was taking most veggie. The seller might know I'm particular with these, and he add 'free' veggie in it after I start questioning how he come out with the price. Here come the funny, when I take back and weight, the whole thing plus the 'free' veggie he added later on, weighted 500g+

Today I'm going to challenge once again, I pick all the veggie, and ask to weight first, as I might add on, the seller a bit reluctant to weigh but after all he told me it was 300g, I add in a bit meats, and ask for weigh, now it cost worth 600g of meats. I ask for explanation how they come out with the price, the seller said he weight it but couldn't explain it. AND THE WEIGHING SCALE is cover behind something so I couldn't see the weight, but I'm sure there are NO FAIR WEIGHT SCALE Mark on it.

At the end the seller don't want to sell me the item, and chase me out. The thing is not about the extra dollars I paid last time, but I really mad at this kind of arrogant dishonest seller, acting like no one can rule them. Please let me know anything I can do. Thanks!