Control account reconciliations – guidance needed

The balance on the trade receivable control account is 57,852 and the balance of the total of the individual trade receivable is 59,152.

The difference may be due to:

A. entering a discount allowed of 650 on the debit side of the control account

Why is this the correct answer? I assume that when you debit anything in the control account, it would increase in relation to the individual trade receivable account x 2, but this is having the opposite effect.

Any help would be greatly appreciated!