Bex report Variable to get price from DSO1 filtered by DSO2

Hi experts,

I'm creating a query on a Multiprovider (compound by two DSO).

In the first DSO we have Sales invoice.

In the second DSO we have 2 ratios (prices) uploaded by a CSV (Keys: material number and 0calmonth)

The question is the following:

When I create the DSO, i need to create a formula or variable to link the prices by material and date with the invoices.

Any ideas on how could i achieve this?

Thanks in advanced,

Florencia