usually entry for posting on sales account is to give debit to customer and give credit to sales account. entry posting for purchase account is to credit the customer and debit the purchases account. purchase account is a asset account and hence posting the debit increases the number of purchases by 1000 which has to be decreased and sales account which is revenue account which is not posted should be increased by the amount of sales which can be done by crediting the sales account by the number of the sales. finally the customer who was credited 1000 instead of being debited 1000 (Its like saying you brought goods from me so I owe you 1000 rupees for the goods brought from me!! so first we have reduce what we owe to him by 1000 and then say that he owes us 1000) thus you debit the customer with 2000
original entries(wrong entries)
purcases Dr. 1000 ----
Ram A/c ------- 1000
correcting entries
(first reverse the wrong entry)
Ram A/c Dr. 1000 ----
purcases ------- 1000
The we write the entry which is ommitted (record sales)
Ram A/c Dr. 1000 ----
sales ------- 1000
the compound entry combining both is
Ram A/c Dr. 2000 ----
purcases ------- 1000
sales ------- 1000