A single cheat meal usually will not ruin a full week of weight loss progress as long as your overall calorie intake stays reasonable.
It can slow your progress if the cheat meal turns into a very large overeating session with thousands of extra calories.
Weight loss depends more on your total calorie balance over time than on one meal.
Eating more carbohydrates and salty foods can make your body hold more water which can temporarily increase your weight.
I think it is better to call it a planned flexible meal instead of treating it like a reward for being good all week.
One high calorie meal can be balanced by returning to your normal eating routine instead of trying to starve yourself the next day.
The actual impact depends on how many extra calories the meal contains and how large your normal weekly calorie deficit is.
Your body weight naturally fluctuates because of water food digestion sodium carbohydrates and other factors.