Hi Sumant,
This is SAP standard, you cannot create any financial entry in SAP with out Currency, however you can default Currency field, just go f-02, click on currency field and press F9, here you can select "Default Currency Option"
To post a document in SAP, we have to pass some basic mandatory fields:
1. document date
2.. Posting Date
3. Company Code
4. Currency
5. Document Type
6. Posting Keys (Debit or Credit), no need to enter posting keys in case of enjoyable transaction, e.g. FB60, FB70, MIRO etc.
7. Amount
8. Account Assignment Fields etc.
Without above you cannot post financial documents in SAP system.
Regards
Javed