I have receipted the incorrect amount onto a sales ledger account, how do I correct?

In the below example we have posted £690 as a receipt to a sales ledger account, but this amount should have actually been for £790.

The best way to correct in these instances is to reverse the receipt in full, then post the correct amount again. 

To do this go into Sales Ledger > Receipt and enter the exact value you posted, but as a negative, then press Allocate Cash

Then press Post and Complete Batch.

On the account you will now have the original cash posting, and the reversal. It is now safe to post the correct receipt value.