Assign a value to first linked record only

When using linked records, is it possible to apply a value to only the first linked record in a cell?

So if I have two tables.

Table A

Table B

With Table A, I'd like to find a way for the Revenue to only be a logged against the first linked record in the Products column. Is there a way to do this?

(I can't do it the other way around - so applying the rollup to the first linked record only in the Sales column on table B, since I do need to be able to sum multiple Sales in this column). 

Effectively, I need to be able to roll up revenue from sales against each product, but in cases where multiple products are placed against a sale in Table A, I only want the revenue to be assigned to the first product in the list. 

Hope that makes sense. 

Thanks.