Decrease in trade payables exercise

Select figures from a firm’s budget for next month are as follows.

Sales – $450000
Gross profit on sales – 30%
Decrease in trade payables over the month – $10000
Increase in cost of inventory held over the month – $18000

What is the budgeted payment to trade payables?

Answer
payment=$(450000×70%+18000+10000)= $343000

My problem is that I understand everything except adding the 10000.
I would think that decrease in payables would mean less of an expense, right?

So why are we adding it to our payments?

Any help is appreciated!