Incorrect transaction fee estimation when payout

Question

I have been working on payout in my L0 relay chain blockchain .While making payout I have observed the transaction fee that is deducted from my account and the actual fee that is been deducted are different, what I see from events the extra deducted fee is deposited back to my account. I want to know why extra fee get deducted from my account and then deposited back again.

eg. lets suppose i have made payout transaction from A account.

1. 0.2 units fee gets deducted

2. 0.18units gets credited to my account.

3. 0.02units fee gets deducted this way.

I am sharing the screenshot for the same.

Answer 1

I assume what you did was calling Staking::payout_stakers. This call first deduces the worst-case fee and then refunds all amount that was not used.
This is a standard pattern in Substrate when only the worst-case effort is known in advance.
The refund happens here where a DispatchResultWithPostInfo is returned that corrects the consumed weight downwards. Note that it is only possible to reduce it downwards - not upwards, since the provided pre-dispatch weight must always be the worst-case.