Why does the addition of a new key-value pair to a big_map not incur a storage fee at all times?

Question

For example- I am sending an FA1.2/FA2 token to an address that has previously not held any amount of that token. So, the address is not present in the ledger big_map of the contract. This would clearly lead to an increase in storage being used, since the address and the corresponding balance needs to be added to the big_map. Mostly this incurs a storage fee ~0.01675 tez (on mainnet).

However, at times, I have noticed that no storage fee is incurred in such a scenario (just gas). Why does this seem to happen?

Answer 1

That may happen when someone removes data from the storage before your operation, releasing some space, so that the contract has a kind of "prepaid" space, for which storage fee won't be taken.

It works like this:

• op1 writes 10 bytes (storage fee is taken for 10 bytes);
• op2 removes 5 bytes (no storage fees, nor refunds);
• op3 writes 3 bytes (no storage fees);
• op4 writes 3 bytes (storage fee is taken for 1 byte).