Florence,
I always recommend testing this for yourself, but I am pretty sure than unless you are tracking serial numbers or lots for items, you cannot be certain that the cost on the sales transaction will be taken from the cost on the linked PO.
In my testing, when you are not using serial numbers or lots, regardless of the cost on the linked PO, GP will take the first cost layer available if you're using FIFO or the average cost when you are using average perpetual.