**Rectification Entries for Omitted Discount Transaction**

To rectify the error of omitting the discount allowed by Mr. Y on 25th June, 2017, we need to make the necessary rectification entries in the accounts. The omitted discount amount of Rs. 684 needs to be posted in Mr. Y's account to reflect the accurate financial position and to ensure the books of accounts are correct.

Here are the rectification entries:

1. Rectification Entry in Mr. Y's Account:
- Debit: Discount Allowed Account
- Credit: Mr. Y's Account

This entry is made to account for the discount allowed by Mr. Y, which was previously omitted. The discount allowed account is debited to increase the discount expense, while Mr. Y's account is credited to reflect the reduction in his outstanding balance.

2. Rectification Entry in Discount Allowed Account:
- Debit: Mr. Y's Account
- Credit: Discount Allowed Account

This entry is made to reverse the previous omission of the discount allowed. Mr. Y's account is debited to reduce his outstanding balance, while the discount allowed account is credited to decrease the discount expense.

3. Rectification Entry in Purchase Account:
- Debit: Discount Allowed Account
- Credit: Purchase Account

This entry is made to adjust the purchase account, as the discount allowed should have been deducted from the purchase amount. The discount allowed account is debited to decrease the discount expense, while the purchase account is credited to reduce the recorded purchase amount.

These rectification entries will ensure that the accounts reflect the accurate financial position by properly accounting for the omitted discount transaction. It is important to rectify such errors promptly to maintain the integrity and accuracy of the financial statements.