Need advice on a specific case of customer owned inventory

Hi to all

We are a small manufacturing company that produce door panels and construction panels.

We are facing now a situation where some important customer want us to keep some inventory for semi elaborated goods in order that they can have shorter delivery times

The exact situation is that in a process like this

Prime Material A -> (process 1) -> Semi elaborated good B -> (process 2) -> finished product C

Now the ask us to purchase us directly Semi elaborated good B, but keep that good in our stock, in a way that we can finish the product faster when they make a new order