ROAS divides attributed sales by ad spend. To know what is left, first work out the contribution of those sales after product cost and other variable costs.
An example with numbers
A campaign costs $1,000 and is credited with $4,000 of sales. Its ROAS is 4. With a 20% contribution before advertising, those sales leave $800. After paying for the campaign, $200 is missing, even assuming every sale is incremental.
Try it with your numbers
Work out the minimum ROAS that covers advertising at that margin: 1 ÷ 0.20 = 5. Then check how many sales would have happened anyway.
Example figures for learning purposes. They are not a recommendation and do not replace professional advice.