The balance in the supplies account before adjustment at the end of the year is $873. The proper adjusting entry if the amount of supplies on hand at the end of the year is $270 would be

The proper adjusting entry is as follows

Supplies expense A/c Dr $603

To Supplies A/c $603

(Being supplies account is adjusted)

The supplies expense is computed by

= Supplies account balance - supplies on hand at the end of the year

= $873 - $270

= $603

Basically we debited the supplies expense account and credited the supplies account so that the proper posting could be done.